- don't need multiple prepare steps
- try to use git output for SHA and log message

- expression context is not enough

- unclear why this wasn't evaluated in expression context before

- this should prevent forks from duplicate-reporting builds

FIXES #1631

modules/bootloader: Use the correct names for the shim binaries

Ever since signed shim binaries for multiple architectures became
available, the shim binaries installed in Linux distributions have
been renamed to include the EFI architecture in the binary names.

This started in Fedora, but is now used in openSUSE and Ubuntu too.

Reference for shim binary names comes from shim spec in Fedora:

https://src.fedoraproject.org/rpms/shim/blob/d8c3c8e39235507feb17b4e81851da087ae83c77/f/shim.spec#_23-32