[Rust] Correct the gcov path prefix.

strings.TrimPrefix() args are reversed, leading to an incorrect gcov
path prefix for Rust coverage.

Bug: 156482307
Test: built and ran unicode-xid_device_tests_unicode_xid with coverage
Change-Id: I6476cee14093a7e53738019c84dbcf4333b0b9b2
diff --git a/rust/builder.go b/rust/builder.go
index 5069b07..7dbb59d 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -163,7 +163,7 @@
 	if flags.Coverage {
 		var gcnoFile android.WritablePath
 		// Provide consistency with cc gcda output, see cc/builder.go init()
-		profileEmitArg := strings.TrimPrefix("PWD=", cc.PwdPrefix()) + "/"
+		profileEmitArg := strings.TrimPrefix(cc.PwdPrefix(), "PWD=") + "/"
 
 		if outputFile.Ext() != "" {
 			gcnoFile = android.PathForModuleOut(ctx, pathtools.ReplaceExtension(outputFile.Base(), "gcno"))