srcfiles: Fix compile with --disable-libdebuginfod

srcfiles: Fix compile with --disable-libdebuginfod

The eu-srcfiles --no-backup option is only valid when build with
libdebuginfod support. Adjust the ENABLE_LIBDEBUGINFOD conditional
to not use no_backup anywhere. Also only run the run-srcfiles-self.sh
test when debuginfod is enabled.

     * src/srcfiles.cxx (zip_files): Move endif ENABLE_LIBDEBUGINFOD
     after no_backup check.
     * tests/Makefile.am (TESTS): Only add run-srcfiles-self.sh
     if DEBUGINFOD and !DUMMY_LIBDEBUGINFOD.
     * tests/run-srcfiles.self.sh: Use local DEBUGINFOD_CACHE_PATH.

https://sourceware.org/bugzilla/show_bug.cgi?id=32155

Signed-off-by: Mark Wielaard <mark@klomp.org>