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

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