gnome.updateScript: More Python

This was prompted by the need to capture exit status of `find-latest-version.py` in the next commit.
I had to drop `errexit` (if I did not want to result to even worse hacks) but that concealed errors like the following,
when I accidentally used an incorrect equals operator in numeric comparison:

    line 24: ((: 1 = 1 : attempted assignment to non-variable (error token is "= 1 ")

Converting the plumbing in `gnome/update.nix` to Python also makes it slightly easier to read.

For now, `find-latest-version.py` is still invoked as a separate process (rather than being imported as a Python module),
as `update.nix` might be replaced by `genericUpdater` in the future.

pkgs/desktops/gnome/update.nix

@@ -1,4 +1,4 @@
-{ stdenv, bash, pkgs, lib, writeScript, python3, common-updater-scripts }:
+{ stdenv, pkgs, lib, writeScript, python3, common-updater-scripts }:
 { packageName, attrPath ? packageName, versionPolicy ? "tagged", freeze ? false }:
 
 let
@@ -20,24 +20,54 @@ let
     else
       throw "“freeze” argument needs to be either a boolean, or a version string.";
   updateScript = writeScript "gnome-update-script" ''
-    #!${bash}/bin/bash
-    set -o errexit
-    attr_path="$1"
-    package_name="$2"
-    package_version="$3"
-    version_policy="$4"
+    #!${python}/bin/python
+    import json
+    import os
+    import subprocess
+    import sys
 
-    flvFlags=("$package_name" "$version_policy" "''${GNOME_UPDATE_STABILITY:-stable}")
+    _, attr_path, package_name, package_version, version_policy, *remaining_args = sys.argv
 
-    if (( $# >= 5 )); then
-      upper_bound="$5"
-      flvFlags+=("--upper-bound=$upper_bound")
-    fi
+    flv_args = [
+        package_name,
+        version_policy,
+        os.environ.get("GNOME_UPDATE_STABILITY", "stable"),
+    ]
 
-    PATH=${lib.makeBinPath [ common-updater-scripts python ]}
-    latest_tag=$(python "${./find-latest-version.py}" "''${flvFlags[@]}")
-    update-source-version "$attr_path" "$latest_tag"
-    echo '[ { "commitBody": "https://gitlab.gnome.org/GNOME/'$package_name'/-/compare/'$package_version'...'$latest_tag'" } ]'
+    match remaining_args:
+        case []:
+            pass
+        case [upper_bound]:
+            flv_args.append(f"--upper-bound={upper_bound}")
+        case other:
+            print("gnome-update-script: Received too many arguments.", file=sys.stderr)
+            sys.exit(1)
+
+    latest_tag = subprocess.check_output(
+        [
+            "${python}/bin/python",
+            "${./find-latest-version.py}",
+            *flv_args,
+        ],
+        encoding="utf-8",
+    )
+    latest_tag = latest_tag.strip()
+    subprocess.run(
+        [
+            "${common-updater-scripts}/bin/update-source-version",
+            attr_path,
+            latest_tag,
+        ],
+        check=True,
+    )
+
+    report = [
+        {
+            "commitBody": f"https://gitlab.gnome.org/GNOME/{package_name}/-/compare/{package_version}...{latest_tag}",
+        },
+    ]
+
+    print(json.dumps(report))
   '';
 in {
   name = "gnome-update-script";