Use our nixpkgs revision for the start script too

This avoids pulling down a different ghc version than is used for the tests on CI.

rules_haskell_tests/tests/run-start-script.sh

@@ -15,6 +15,26 @@ mkdir $workdir
 cd $workdir
 cp "$rules_haskell_dir/.bazelversion" .
 
+function getattr_value() {
+    local nix_file="$rules_haskell_dir/nixpkgs/default.nix"
+    while IFS=$' \t;' read -r key eq value rest ; do
+        if [ "$key" == "$1" ] && [ "$eq" == '=' ]; then
+            value="${value%\"}"
+            value="${value#\"}"
+
+            echo "$value"
+            return
+        fi
+    done < "$nix_file"
+    echo "could not lookup ${1} in $nix_file" >&2
+    exit 1
+}
+
+NIXPKGS_REVISION=$( getattr_value "rev" )
+NIXPKGS_HASH=$( nix hash to-base64 "$(getattr_value "sha256")" )
+
+export NIXPKGS_REVISION NIXPKGS_HASH
+
 # specify version for bazelisk via `USE_BAZEL_VERSION`, since it does not read the .bazelversion when there is no WORKSPACE file
 USE_BAZEL_VERSION=$( cat .bazelversion )
 export USE_BAZEL_VERSION

start

@@ -9,6 +9,8 @@ set -eu
 # we use the default version (currently "9.4.6").
 GHC_VERSION=${GHC_VERSION:="9.4.6"}
 
+NIXPKGS_REVISION=${NIXPKGS_REVISION:-nixos-24.05}
+
 readonly MIN_BAZEL_MAJOR=6
 readonly MIN_BAZEL_MINOR=0
 
@@ -328,7 +330,8 @@ EOF
 		# https://github.com/tweag/rules_nixpkgs/blob/master/README.md
 		nixpkgs_git_repository(
 		    name = "nixpkgs",
-		    revision = "nixos-24.05",
+		    revision = "${NIXPKGS_REVISION}",${NIXPKGS_HASH:+
+		    sha256 = \"$NIXPKGS_HASH\",}
 		)
 
 		nixpkgs_cc_configure(