reprotest: Add --append-build-command option
This MR rebases the work that @ina did in !381 (closed) against the master branch and addresses this comment[1] around enclosing in quotes.
If $SALSA_CI_DPKG_BUILDPACKAGE_ARGS is set, include the --append-build-command CLI option with $SALSA_CI_DPKG_BUILDPACKAGE_ARGS value.
Closes: #245 (closed) Signed-off-by: Lev Lazinskiy lev@levlaz.org