[Test][ScanDependencies] Use env for variable assignment

Author: ramonasuncion

test/ScanDependencies/direct-cc1-dwarf-options.swift

@@ -2,7 +2,7 @@
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t
 
-// RUN: RC_DEBUG_OPTIONS=1 %target-swift-frontend -scan-dependencies -o %t/deps.json -I %t \
+// RUN: env RC_DEBUG_OPTIONS=1 %target-swift-frontend -scan-dependencies -o %t/deps.json -I %t \
 // RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
 // RUN:   -g -file-compilation-dir %t -Xcc -ferror-limit=1 \
 // RUN:   %t/test.swift -module-name Test -swift-version 5 -experimental-clang-importer-direct-cc1-scan