Automate the creation of a nightly-as-release variant

[tysmith]

We will need to scan m-c (StaticPrefList.yaml, firefox.js and all.js) for prefs with the value of @IS_NIGHTLY_BUILD@ and @IS_EARLY_BETA_OR_EARLIER@.
These can then be added to a variant that with the prefs set to false.

This will enable fuzzing of nightly builds which closely match the available feature set of release builds.

[dholbert]

We will need to scan m-c (StaticPrefList.yaml, firefox.js and all.js) for prefs with the value of @IS_NIGHTLY_BUILD@ and @IS_EARLY_BETA_OR_EARLIER@.
These can then be added to a variant that with the prefs set to false.

Same goes for the NOT versions of these IS_NOT_NIGHTLY_BUILD and IS_NOT_EARLY_BETA_OR_EARLIER - those evaluate to false on Nightly and true on release, so we'd want our variant to have all these prefs set to true in order to emulate a release config.