RFR: Fix gradlew on Cygwin

[Erik Helin]

Hi all,

this small patch fixes the Skara build on Cygwin _if_ `gradlew` is used instead of `gradlew.bat`. There is no reason for `Unzip.java` in the Cygwin case since the Windows JDK is itself a ZIP archive, so we need a native `unzip` to pack up the Windows JDK. Therefore we might as well use the native `unzip` when unpacking Gradle as well.

## Testing
- `sh gradlew` works in Cygwin on Windows

Thanks,
Erik

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Commits:
 - 64cf0399: Fix gradlew on Cygwin

Changes: https://git.openjdk.java.net/skara/pull/196/files
 Webrev: https://webrevs.openjdk.java.net/skara/196/webrev.00
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/196.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/196/head:pull/196

PR: https://git.openjdk.java.net/skara/pull/196