[llvm] Add env prefix to environment variable

This substitution is used in a few tests in certain build configurations
(seems like only a 2-stage build on Darwin with Asan enabled on a
previous stage). This needs an env prefix now that we have enabled the
internal shell by default, or the tests end up failing.

Author: boomanaiden154

llvm/test/lit.cfg.py

@@ -156,7 +156,7 @@ def get_asan_rtlib():
 ld64_cmd = config.ld64_executable
 asan_rtlib = get_asan_rtlib()
 if asan_rtlib:
-    ld64_cmd = "DYLD_INSERT_LIBRARIES={} {}".format(asan_rtlib, ld64_cmd)
+    ld64_cmd = "env DYLD_INSERT_LIBRARIES={} {}".format(asan_rtlib, ld64_cmd)
 if config.osx_sysroot:
     ld64_cmd = "{} -syslibroot {}".format(ld64_cmd, config.osx_sysroot)