Arm backend: Fix mypy linting in pre-push

- 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 <[email protected]>
Change-Id: Ia7ce8f02e1ae526dce91758bc267cd78f1f40ee6

Author: AdrianLundell

backends/arm/scripts/pre-push

@@ -76,7 +76,7 @@ for COMMIT in ${COMMITS}; do
 
     # lintrunner on latest patches.
     echo -e "${INFO} Lintrunner"
-    lintrunner --revision ${COMMIT}
+    MYPYPATH=./src/ lintrunner --revision ${COMMIT}^1
     if [[ $? != 0 ]]; then
         echo -e "${ERROR} Failed linting"
         FAILED=1