CI: use bash for output-sizs-job

Firmware build works. But the build size job afterwards fails.
The firmware build uses `shell: bash --noprofile --norc -e -o pipefail {0}` as shell.

The size job uses `shell: sh -e {0}`

The variable substitution I introduced are bash features. So they don't work with sh.

Update the size job to use `bash` instead of `sh` as shell

.github/workflows/main.yml

@@ -44,6 +44,7 @@ jobs:
           /opt/build.sh all
       - name: Output build size
         id: output-sizes
+        shell: bash
         run: |
           . /opt/build.sh
           .github/workflows/getSize.sh "$BUILD_DIR"/src/pinetime-app-*.out >> $GITHUB_OUTPUT