ci: don't hardcode ~/.local/bin when running pre-commit

The pre-commit job failed with

    /root/.local/bin/pre-commit: No such file or directory

even though `pipx install pre-commit` succeeded: `pipx install` writes the
app to PIPX_BIN_DIR, and the runner image no longer uses ~/.local/bin for
it. Run the binary from PATH when pipx put it there, and fall back to
`pipx run pre-commit` otherwise, so the hook still runs regardless of
where pipx exposes its apps.

.gitea/workflows/ci.yml

@@ -12,7 +12,15 @@ jobs:
         run: pipx install pre-commit
 
       - name: Run pre-commit
-        run: ~/.local/bin/pre-commit run --all-files --show-diff-on-failure
+        # `pipx install` writes the app to PIPX_BIN_DIR, which is not
+        # necessarily ~/.local/bin and not necessarily on PATH, so don't
+        # hardcode a path that only some runner images happen to use.
+        run: |
+          if command -v pre-commit >/dev/null 2>&1; then
+            pre-commit run --all-files --show-diff-on-failure
+          else
+            pipx run pre-commit run --all-files --show-diff-on-failure
+          fi
 
   build:
     strategy: