FGCP: read FGCP_CERT_DIR env var in method instead of start of class

With this, fgcp tests pass again on MRI1.9.3.
PR addresses the issue I described in the ML, titled 'unit tests failing on fgcp - ENV not propagated?'.
I was using an ENV var to override the location of certificates. This was used by the unit tests. Last month a change was made to preload drivers, which probably broke this: at the time the test sets the ENV var, the fgcp driver has already been loaded and the ENV var was (unsuccessfully) read. Moved the ENV var check into the method that uses it.