build: change environment variables immediately inside __init__()

Otherwise various recipe.get_env() calls inside a recipe's prepare() will not retrieve the correct value. Fixes bitcode bundling in at least rtmp and the MAKEFLAGS/CONFIG_SITE overriding.

We also need to pass the environment to be used earlier (on creation) for this to work.

Contains an environment hack to not add PKG_CONFIG_PATH from the system when compiling so that gobject-introspection does not pick up the incorrect libraries.

Fixes:

ld: '/Users/gst-ci/Library/Developer/GStreamer/iPhone.sdk/GStreamer.framework/GStreamer(librtmp_a_armv7_-log.o)' does not contain bitcode. You must rebuild it with bitcode enabled (Xcode setting ENABLE_BITCODE), obtain an updated library from the vendor, or disable bitcode for this target. for architecture armv7