add -I key=value option

Allow to inject other NIX_PATH search values like nixpkgs.

nixos-generate

@@ -4,13 +4,17 @@ set -euo pipefail
 
 ## Configuration
 
-libexec_dir="${0%/*}"
+readonly libexec_dir="${0%/*}"
+readonly format_dir=$libexec_dir/formats
+
 configuration=${NIXOS_CONFIG:-$libexec_dir/configuration.nix}
 format_path=
-format_dir=$libexec_dir/formats
 target_system=
 cores=
 run=
+nix_args=(
+  "$libexec_dir/nixos-generate.nix"
+)
 
 ## Functions
 
@@ -30,6 +34,7 @@ Options:
          only works for the "vm" and "vm-no-gui" formats
 * --system: specify the target system (eg: x86_64-linux)
 * --cores : to control the maximum amount of parallelism. (see nix-build documentation)
+* -I KEY=VALUE: Add a key to the Nix expression search path.
 USAGE
 }
 
@@ -83,6 +88,10 @@ while [[ $# -gt 0 ]]; do
       target_system=$2
       shift
       ;;
+    -I)
+      nix_args+=(-I "$2")
+      shift
+      ;;
     *)
       abort "unknown option $1"
       ;;
@@ -94,23 +103,22 @@ if [[ -z $format_path ]]; then
   abort "missing format. use --help for more details"
 fi
 
-args=(
-  "$libexec_dir/nixos-generate.nix"
+nix_args+=(
   -I "nixos-config=$configuration"
   -I "format-config=$format_path"
 )
 
 if [[ -n $target_system ]]; then
-  args+=(--argstr system "$target_system")
+  nix_args+=(--argstr system "$target_system")
 fi
 
 if [[ -n $cores ]]; then
-  args+=(--cores "$cores")
+  nix_args+=(--cores "$cores")
 fi
 
-formatAttr=$(nix-instantiate "${args[@]}" --eval --json -A config.formatAttr | jq -r .)
+formatAttr=$(nix-instantiate "${nix_args[@]}" --eval --json -A config.formatAttr | jq -r .)
 
-out=$(nix-build "${args[@]}" --no-out-link -A "config.system.build.$formatAttr")
+out=$(nix-build "${nix_args[@]}" --no-out-link -A "config.system.build.$formatAttr")
 
 if [[ -z $run ]]; then
   # show the first file