build: add explicit --no-show-signature for git

build: add explicit --no-show-signature for git

When `log.showSignature` is set, it causes the `SOURCE_DATE_EPOCH` to
include a textual signature description on OpenPGP-signed commits,
because Git prints the description into stdout. This then causes some
scripts to fail because they cannot parse the date from the variable.

Adding an explicit `--no-show-signature` prevents the signatures from
being displayed even when one has Git configured to show them by
default, fixing the scripts.

Signed-off-by: Oto Šťáva <oto.stava@gmail.com>