meson: use files in run_command with relativized path

meson: use files in run_command with relativized path

Passing a file as a command argument in string form assumes that
run_command has the current subdir as its cwd, but Meson's documentation
*explicitly* calls this out as undefined and wrong to use.

Indeed, muon has a different implementation that uses a different cwd,
and this argument cannot be found. Instead, passing a files() object
means that it's the job of meson itself to verify the file exists, then
pass it to the run_command in some format that guarantees it is a valid
path reference.