Rollup merge of rust-lang#146140 - jeremyd2019:patch-1, r=jieyouxu

compiletest: cygwin follows windows in using PATH for dynamic libraries

```@Berrysoft```

Author: jhpratt

src/tools/compiletest/src/util.rs

@@ -45,7 +45,7 @@ impl Utf8PathBufExt for Utf8PathBuf {
 
 /// The name of the environment variable that holds dynamic library locations.
 pub fn dylib_env_var() -> &'static str {
-    if cfg!(windows) {
+    if cfg!(any(windows, target_os = "cygwin")) {
         "PATH"
     } else if cfg!(target_vendor = "apple") {
         "DYLD_LIBRARY_PATH"