gitlab-ci: do not regenerate the uroot container by default

1 job for master in 1 minute and 3 seconds
Status Job ID Name Coverage
  Tests
failed #8223055
tests

00:01:03

 
Name Stage Failure
failed
tests Tests
2021/03/23 12:08:29 WARNING: You are not using one of the recommended Go versions (have = go1.16.2, recommended = [go1.13 go1.14]).
Some packages may not compile.
Go to https://golang.org/doc/install to find out how to install a newer version of Go,
or use https://godoc.org/golang.org/dl/go1.13 to install an additional version of Go.
2021/03/23 12:08:29 Filename is /tmp/initramfs.linux_amd64.cpio
2021/03/23 12:08:29 "github.com/u-root/u-root/cmds/core/init" is neither package or path/glob: no required module provides package github.com/u-root/u-root/cmds/core/init: go.mod file not found in current directory or any parent directory; see 'go help modules'
make: *** [Makefile:39: out/initramfs.linux_amd64.cpio] Error 1
Cleaning up file based variables
ERROR: Job failed: exit code 2