utils: Filename: Prepend the mrl with a / before encoding it

So the logic to avoid encoding ':' when part of a drive letter can kick
in
4 jobs for fix_win32_mrl_encode in 12 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #285994
amd64 debian
asan-ubsan

00:10:39

passed #285991
amd64 debian
linux

00:08:26

passed #285992
win32
win32

00:11:56

passed #285993
win64
win64

00:11:37