Append .bash to completions

[oech3]

Closes scop/bash-completion#1385

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[sylvestre]

i didn't know it was a thing now:

$ ls /usr/share/bash-completion/completions/*bash
/usr/share/bash-completion/completions/alacritty.bash
/usr/share/bash-completion/completions/fd.bash
/usr/share/bash-completion/completions/ibus.bash
/usr/share/bash-completion/completions/sqv.bash