crypt: make it possible to compile and test on macOS

Our current crypt implmentation requires glibc to be available on the
developer's system. But it is not strictly necessary, because I'd like
to run at least unit tests on mac. This PR makes it possible to compile
and run unit-tests on mac, but it will panic if someone tries to run
osbuild-composer on mac.

internal/crypt/crypt_impl.go

@@ -1,3 +1,5 @@
+// +build !macos
+
 // Copied from https://github.com/amoghe/go-crypt/blob/b3e291286513a0c993f7c4dd7060d327d2d56143/crypt_r.go
 // Original sources are under MIT license:
 // The MIT License (MIT)

internal/crypt/crypt_impl_macos.go

@@ -0,0 +1,8 @@
+// +build macos
+
+package crypt
+
+func crypt(pass, salt string) (string, error) {
+	panic("You must not run osbuild-composer on macOS!")
+	return "", nil
+}