manibuilder: Replace `master` branch with `main`

Change-Id: I4a7c6d0238cf915473cd5a2143e6afc00817caac
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73431
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed