firefox{-esr,}: fix failing build due to the google-api-key

Since firefox 58.0.1 the google api key is now stored at an absolute
path ($TMPDIR/ga). Since variable expansion in `configureFlags` does not
really work (as expected) the build started failing when using the
legacy firefox build system. With the newer `./mach` based builds
firefox reads the configure flags from `.mozconfig` instead.

This commit moves the `with-google-api-keyfile=` setting into the
`preConfigure` phase where we can properly expand `$TMPDIR` into
whatever the path is.