CI: Speed up static checks by checking only changed files

- file_format, header_guards and clang-format benefit from this short list.
- dotnet-format, Python and JS checks don't, but they're only relevant for
  PRs changing a specific set of files, so we skip them when those files
  aren't modified.

The logic to get changed files only works reliably for:
- Pull request events
- Non-force pushed push events

So when force pushing a branch in your fork, or creating a new branch,
it will still scan all files as fallback.

Upgraded CI runner to Ubuntu 22.04 so we get clang-format 14 out of the box,
so we don't need to install a custom version (saves ~15 s). We also cache
the APT dependencies to speed up the build and avoid flaky Ubuntu/Microsoft
repos.

misc/scripts/file_format.sh

@@ -7,14 +7,20 @@
 # We need dos2unix and isutf8.
 if [ ! -x "$(command -v dos2unix)" -o ! -x "$(command -v isutf8)" ]; then
     printf "Install 'dos2unix' and 'isutf8' (moreutils package) to use this script.\n"
+    exit 1
 fi
 
 set -uo pipefail
-IFS=$'\n\t'
 
-# Loops through all text files tracked by Git.
-git grep -zIl '' |
-while IFS= read -rd '' f; do
+if [ $# -eq 0 ]; then
+    # Loop through all code files tracked by Git.
+    mapfile -d '' files < <(git grep -zIl '')
+else
+    # $1 should be a file listing file paths to process. Used in CI.
+    mapfile -d ' ' < <(cat "$1")
+fi
+
+for f in "${files[@]}"; do
     # Exclude some types of files.
     if [[ "$f" == *"csproj" ]]; then
         continue