Lint .desktop files in CI

This ports the `lint-desktop-files` CI job from the securedrop-builder
repository.

.github/workflows/ci.yml

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+name: CI
+on: [push, pull_request]
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  lint-desktop:
+    strategy:
+      matrix:
+        debian_version:
+          - bullseye
+          - bookworm
+    runs-on: ubuntu-latest
+    container: debian:${{ matrix.debian_version }}
+    steps:
+      - run: |
+          apt-get update && apt-get install --yes git make desktop-file-utils
+      - uses: actions/checkout@v4
+      - name: Lint .desktop files
+        run: |
+          make lint-desktop

Makefile

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+.PHONY: all
+all: help
+
+.PHONY: lint-desktop
+lint-desktop: ## Lint .desktop files
+	find . -name *.desktop -type f -not -path '*/\.git/*' | xargs desktop-file-validate
+
+
+# Explanation of the below shell command should it ever break.
+# 1. Set the field separator to ": ##" and any make targets that might appear between : and ##
+# 2. Use sed-like syntax to remove the make targets
+# 3. Format the split fields into $$1) the target name (in blue) and $$2) the target description
+# 4. Pass this file as an arg to awk
+# 5. Sort it alphabetically
+# 6. Format columns with colon as delimiter.
+.PHONY: help
+help: ## Print this message and exit.
+	@printf "Makefile for developing and testing the SecureDrop client.\n"
+	@printf "Subcommands:\n\n"
+	@awk 'BEGIN {FS = ":.*?## "} /^[0-9a-zA-Z_-]+:.*?## / {printf "\033[36m%s\033[0m : %s\n", $$1, $$2}' $(MAKEFILE_LIST) \
+		| sort \
+		| column -s ':' -t