cmake: yaml: update timestamp on intermediate file save

file(GENERATE ...) does not update the output file if the content is
unchanged. Since the metadata in build_info.yml mostly depends on the
build configuration, the timestamp of the intermediate file does not get
updated on most rebuilds, while the final file does, due to immediate
file(WRITE ...) calls. Since the latter is newer, no post-process step
is executed and the file is left with commented genexes.

Touching the intermediate file ensures that the post-process step is
performed every time, even if the content is unchanged, restoring the
expected behavior.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>