Don't cache $wgResourceLoaderLESSVars in a static

Caching the value of $wgResourceLoaderLESSVars in a static variable that is
enclosed in a closure makes it harder to reset global state for test runs, and
it does so in the name of a performance benefit that is miniscule.

Change-Id: I0958b03818f56ab73c8c9124daa9e54cc59f2428
(cherry picked from commit a2bf2346cdaa4b4dce7b005f20220c3ceb56f6bb)
Merge "Don't cache $wgResourceLoaderLESSVars in a static" into REL1_23