Arm backend: Fix mypy linting in pre-push (#10850)

- Adds ./src/ to MYPYPATH to find imports for editable install
- Make --revision point to the commit before the commit to be linted.
Since lintrunner compares files diffing between --revision and HEAD,
pointing it to the the latest commit (== HEAD) results in no files being
linted.

Signed-off-by: Adrian Lundell <adrian.lundell@arm.com>