build.py: specify commit & (no) APK for easy building/testing

Extend `scripts/build.py -v podman <appid>:<tag>`: - [x] `<appid>:<tag>:[<commit>]:[<apk>|none]` - [x] `commit` overrides the commit checked out (unless empty) - [x] `apk` overrides the APK downloaded (unless empty) - [x] can be a local file path instead of a http(s) URL for local testing - [x] can be `none` to only build, skip RB checks - [x] enable with e.g. `--local` - [x] README++ - [ ] TBD: load dependencies like `apksigcopier`, `requests` on demand? Examples: - `scripts/build.py -v podman me.hackerchick.catima:v2.27.0::none` (local build using tag, no RB checks) - `scripts/build.py -v podman me.hackerchick.catima:v2.27.0:7c7a05f:/some/dir/some.apk` (verify commit against local APK, using recipe from tag) ~~Feedback on the idea before I implement it~~ testing is appreciated :)