fix: set up the environment based on the installer shell scripts

The installer scripts would normally set up the various environment variables that Nix needs.
Since GitHub doesn't run any profile or rc scripts by default, we need to set these up ourselves.

install-nix.sh

@@ -31,6 +31,7 @@ add_config() {
 add_config "show-trace = true"
 # Set jobs to number of cores
 add_config "max-jobs = auto"
+# TODO: is this still necessary if we set NIX_SSL_CERT_FILE below?
 if [[ $OSTYPE =~ darwin ]]; then
   add_config "ssl-cert-file = /etc/ssl/cert.pem"
 fi
@@ -70,8 +71,14 @@ installer_options=(
   --nix-extra-conf-file "$workdir/nix.conf"
 )
 
-# only use the nix-daemon settings if on darwin (which get ignored) or systemd is supported
+# Enable daemon on macOS and Linux systems with systemd, unless --no-daemon is specified
 if [[ (! $INPUT_INSTALL_OPTIONS =~ "--no-daemon") && ($OSTYPE =~ darwin || -e /run/systemd/system) ]]; then
+  use_daemon() { true; }
+else
+  use_daemon() { false; }
+fi
+
+if use_daemon; then
   installer_options+=(
     --daemon
     --daemon-user-count "$(python3 -c 'import multiprocessing as mp; print(mp.cpu_count() * 2)')"
@@ -107,19 +114,70 @@ done
 
 sh "$workdir/install" "${installer_options[@]}"
 
-# Set paths
+# Configure the environment
-echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
+#
-# new path for nix 2.14
+# Adapted from the single- and multi-user scripts:
-echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
+#   single-user: https://github.com/NixOS/nix/blob/master/scripts/nix-profile-daemon.sh.in
+#   multi-user: https://github.com/NixOS/nix/blob/master/scripts/nix-profile-daemon.sh.in
+#
+# These scripts would normally be evaluated as part of the user's shell profile.
+# GitHub doesn't evaluate profiles or rc scripts by default, so we set up the environment manually.
+echo "::debug::Nix installed, setting up environment"
 
+# Export the path to Nix
 if [[ -n "${INPUT_NIX_PATH:-}" ]]; then
   echo "NIX_PATH=${INPUT_NIX_PATH}" >> "$GITHUB_ENV"
 fi
 
-# Set temporary directory (if not already set) to fix https://github.com/cachix/install-nix-action/issues/197
+# Set temporary directory if not already set
+# Fixes https://github.com/cachix/install-nix-action/issues/197
 if [[ -z "${TMPDIR:-}" ]]; then
   echo "TMPDIR=${RUNNER_TEMP}" >> "$GITHUB_ENV"
 fi
 
+# Determine NIX_LINK path (XDG spec, newer XDG-compliant, or legacy)
+if [[ -n "${XDG_STATE_HOME:-}" && -e "$XDG_STATE_HOME/nix/profile" ]]; then
+  NIX_LINK="$XDG_STATE_HOME/nix/profile"
+elif [[ -e "$HOME/.local/state/nix/profile" ]]; then
+  NIX_LINK="$HOME/.local/state/nix/profile"
+else
+  NIX_LINK="$HOME/.nix-profile"
+fi
+
+# Set Nix profiles
+echo "NIX_PROFILES=/nix/var/nix/profiles/default $NIX_LINK" >> "$GITHUB_ENV"
+
+# Set NIX_SSL_CERT_FILE if not already configured
+if [[ -z "${NIX_SSL_CERT_FILE:-}" ]]; then
+  # Check common SSL certificate file locations
+  if [[ -f "/etc/ssl/certs/ca-certificates.crt" ]]; then # NixOS, Ubuntu, Debian, Gentoo, Arch
+    echo "NIX_SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt" >> "$GITHUB_ENV"
+  elif [[ $OSTYPE =~ darwin && -f "/etc/ssl/cert.pem" ]]; then # macOS
+    echo "NIX_SSL_CERT_FILE=/etc/ssl/cert.pem" >> "$GITHUB_ENV"
+  elif [[ -f "/etc/ssl/ca-bundle.pem" ]]; then # openSUSE Tumbleweed
+    echo "NIX_SSL_CERT_FILE=/etc/ssl/ca-bundle.pem" >> "$GITHUB_ENV"
+  elif [[ -f "/etc/ssl/certs/ca-bundle.crt" ]]; then # Old NixOS
+    echo "NIX_SSL_CERT_FILE=/etc/ssl/certs/ca-bundle.crt" >> "$GITHUB_ENV"
+  elif [[ -f "/etc/pki/tls/certs/ca-bundle.crt" ]]; then # Fedora, CentOS
+    echo "NIX_SSL_CERT_FILE=/etc/pki/tls/certs/ca-bundle.crt" >> "$GITHUB_ENV"
+  elif [[ -f "/usr/local/share/certs/ca-root-nss.crt" ]]; then # FreeBSD
+    echo "NIX_SSL_CERT_FILE=/usr/local/share/certs/ca-root-nss.crt" >> "$GITHUB_ENV"
+  elif [[ -f "/etc/pki/tls/cacert.pem" ]]; then # OpenELEC
+    echo "NIX_SSL_CERT_FILE=/etc/pki/tls/cacert.pem" >> "$GITHUB_ENV"
+  elif [[ -f "/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt" ]]; then # fall back to cacert in default Nix profile
+    echo "NIX_SSL_CERT_FILE=/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt" >> "$GITHUB_ENV"
+  elif [[ -f "$NIX_LINK/etc/ssl/certs/ca-bundle.crt" ]]; then # fall back to cacert in user Nix profile
+    echo "NIX_SSL_CERT_FILE=$NIX_LINK/etc/ssl/certs/ca-bundle.crt" >> "$GITHUB_ENV"
+  fi
+fi
+
+# Set paths based on the installation type
+if use_daemon; then
+  # Multi-user daemon install - add both paths
+  echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
+fi
+# Always add the user profile path
+echo "$NIX_LINK/bin" >> "$GITHUB_PATH"
+
 # Close the log message group which was opened above
 echo "::endgroup::"