Disable firmware user mode helpers

We don't need them and it causes issues for postmarket os where the user
mode helper takes a long time to complete.

Closes: #31
Signed-off-by: Guido Günther <guido.gunther@puri.sm>
2 jobs for firmware in 40 minutes and 59 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build:aarch64:Image #10233
librem5

00:40:59

passed build:aarch64:deb #10234
librem5

00:27:45