Fix: preserve symlinks in ZipArchive compress function (#8250)

This PR fixes #8248, by adding the `y` flag to the `zip` command that is
executed when archiving a package's sources for publication to a
registry on a non-Windows OS.

### Motivation:

Without this change, packages which contain symlinks in their sources
will produce invalid and potentially non-buildable archives when
publishing to a registry, since any symlinks are currently transformed
into duplicate files during the archive creation, instead of being
preserved.

### Modifications:

I've added the following flag to the `zip` call:
>   -y   store symbolic links as the link instead of the referenced file

I've additionally modified the associated `testCompress()` test case,
adding logic to ensure that symlinks are preserved correctly.

### Result:

With this change, the resulting zipfile that is uploaded to a registry
will contain symlinks consistent with the original package sources.

Author: sphanley

Sources/Basics/Archiver/ZipArchiver.swift

@@ -120,7 +120,7 @@ public struct ZipArchiver: Archiver, Cancellable {
             arguments: [
                 "/bin/sh",
                 "-c",
-                "cd \(directory.parentDirectory.underlying.pathString) && zip -r \(destinationPath.pathString) \(directory.basename)",
+                "cd \(directory.parentDirectory.underlying.pathString) && zip -ry \(destinationPath.pathString) \(directory.basename)",
             ]
         )
         #endif

Tests/BasicsTests/Archiver/ZipArchiverTests.swift

@@ -118,6 +118,7 @@ final class ZipArchiverTests: XCTestCase {
              try localFileSystem.createDirectory(dir2)
              try localFileSystem.writeFileContents(dir2.appending("file3.txt"), string: "Hello World 3!")
              try localFileSystem.writeFileContents(dir2.appending("file4.txt"), string: "Hello World 4!")
+             try localFileSystem.createSymbolicLink(dir2.appending("file5.txt"), pointingAt: dir1.appending("file2.txt"), relative: true)
 
              let archivePath = tmpdir.appending(component: UUID().uuidString + ".zip")
              try await archiver.compress(directory: rootDir, to: archivePath)
@@ -154,6 +155,12 @@ final class ZipArchiverTests: XCTestCase {
                  try? localFileSystem.readFileContents(extractedDir2.appending("file4.txt")),
                  "Hello World 4!"
              )
+            
+             XCTAssertTrue(localFileSystem.isSymlink(extractedDir2.appending("file5.txt")))
+             XCTAssertEqual(
+                 try? localFileSystem.readFileContents(extractedDir2.appending("file5.txt")),
+                 try? localFileSystem.readFileContents(extractedDir1.appending("file2.txt"))
+             )
          }
      }
-}
+}