Fix empty token on swift package-registry login (Issue #7453) (#7454)

Fix empty token when adding a new package-registry and adding token
interactively on release builds

### Motivation:

When executing swift package-registry login it asks for a token but
fails to retrieve it properly. It sends an empty string to the registry
server and login therefore fails. (see
#7453)

### Modifications:

I've debugged the code a bit and found out, that somehow `buffer` and
`passwordPtr` don't seem to be holding correct values after the
`readpassphrase` call.

- An easy quick fix is to make sure that `buffer` doesn't get
deallocated by adding smth like `_ = buffer` after the String init. This
works but is not nice.
- My first try was to use `buffer` instead of `passwordPtr` to create
the string. These works, but I remember reading somewhere that `&` can
be quite nasty sometimes.
- I also tried `buffer.withUnsafeMutablePointer`. However
`readpassphrase` seems to not only change the content of `buffer` but
move it. This leads to a runtime failure `Fatal error: Array
withUnsafeMutableBufferPointer: replacing the buffer is not allowed`

### Result:
The buffer is retained, the token is properly parsed and sent to the
server.

Author: pwallrich

Sources/PackageRegistryCommand/PackageRegistryCommand+Auth.swift

@@ -71,12 +71,13 @@ private func readpassword(_ prompt: String) throws -> String {
 
     #if canImport(Darwin)
     var buffer = [CChar](repeating: 0, count: PackageRegistryCommand.Login.passwordBufferSize)
+    password = try withExtendedLifetime(buffer) {
+        guard let passwordPtr = readpassphrase(prompt, &buffer, buffer.count, 0) else {
+            throw StringError("unable to read input")
+        }
 
-    guard let passwordPtr = readpassphrase(prompt, &buffer, buffer.count, 0) else {
-        throw StringError("unable to read input")
+        return String(cString: passwordPtr)
     }
-
-    password = String(cString: passwordPtr)
     #else
     // GNU C implementation of getpass has no limit on the password length
     // (https://man7.org/linux/man-pages/man3/getpass.3.html)