jenkins/ci.jpl: Fix Groovy warning of insecure variable interpolation

Credential-holding vars should be interpolated on the shell, not Groovy,
level. So, the dollar sign prefixes should be quoted in Groovy strings
to be passed verbatim to shell.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I932523ad5d0c4b7cabd2694f3f39f22cafdd5e8c
1 file changed