Fix Errors when running make clean.

This commit fixes errors that occur when running make clean, by making
sure the right working directory is set for the commands that are
failing.
1 job for fix-make-clean in 13 minutes and 15 seconds
latest
Status Job ID Name Coverage
  Test
failed #24450
img

00:13:15

 
Name Stage Failure
failed
img Test
cp: cannot stat 'build/coreboot.elf.gz': No such file or directory
cp: cannot stat 'build/harddrive.bin.gz': No such file or directory
cp: cannot stat 'build/livedisk.iso.gz': No such file or directory
cp: cannot stat 'build/harddrive-efi.bin.gz': No such file or directory
cp: cannot stat 'build/livedisk-efi.iso.gz': No such file or directory
sha256sum: '*': No such file or directory
Makefile:66: recipe for target 'ci-img' failed
make: *** [ci-img] Error 1
ERROR: Job failed: exit code 1