Fix a test on Linux

Author: DougGregor

test/Driver/environment.swift

@@ -2,7 +2,7 @@
 // UNSUPPORTED: windows
 // Apple's "System Integrity Protection" makes this test fail on OS X.
 
-// RUN: %swift_driver -target x86_64-unknown-gnu-linux -L/foo/ -driver-use-frontend-path %S/Inputs/print-var.sh %s LD_LIBRARY_PATH | %FileCheck -check-prefix=CHECK${LD_LIBRARY_PATH+_LAX} %s
+// RUN: %swift_driver -sdk "" -target x86_64-unknown-gnu-linux -L/foo/ -driver-use-frontend-path %S/Inputs/print-var.sh %s LD_LIBRARY_PATH | %FileCheck -check-prefix=CHECK${LD_LIBRARY_PATH+_LAX} %s
 
 // CHECK: {{^/foo/:[^:]+/lib/swift/linux$}}
 // CHECK_LAX: {{^/foo/:[^:]+/lib/swift/linux}}