Allow us to just set RUST_TARGEt ourselves instead of hacking around in mozilla's
weird custom build system...

diff -upr firefox-68.9.0.orig/build/moz.configure/rust.configure firefox-68.9.0/build/moz.configure/rust.configure
--- firefox-68.9.0.orig/build/moz.configure/rust.configure	2020-06-02 22:54:39.982616128 +0200
+++ firefox-68.9.0/build/moz.configure/rust.configure	2020-06-02 23:08:37.656332899 +0200
@@ -345,7 +345,7 @@ def rust_triple_alias(host_or_target):
 
             return None
 
-        rustc_target = find_candidate(candidates)
+        rustc_target = os.environ['RUST_TARGET']
 
         if rustc_target is None:
             die("Don't know how to translate {} for rustc".format(