Only replace first underscore in command

Because we have other variables that sometimes include underscores, replacing
all underscores with colons is problematic. This commit updates the sed call in
the jenkins build script to only replace the first occurence of an underscore
with a colon, instead of all of them.
Before:
`echo 'pl_mock MOCK=x86_64' | sed 's/_/:/g'
pl:mock MOCK=x86:64`
After
`echo 'pl_mock MOCK=x86_64' | sed 's/_/:/1'
pl:mock MOCK=x86_64`
Merge pull request #167 from haus/only_replace_first

Only replace first underscore in command