CI: go back to macos-latest

Now that it seems we found a fix to rogpeppe#200, there is no reason to stick
to macos-11, which will likely be deprecated soon.

Update actions/setup-go to its latest version as well.
The new version uses caching by default, which we do not need.

While here, tidy up the cloneFile docs a bit.

clonefile.go

@@ -5,7 +5,7 @@ package testscript
 
 import "os"
 
-// cloneFile creates to as a hard link to the from file.
+// cloneFile makes a clone of a file via a hard link.
 func cloneFile(from, to string) error {
 	return os.Link(from, to)
 }

clonefile_darwin.go

@@ -2,7 +2,7 @@ package testscript
 
 import "golang.org/x/sys/unix"
 
-// cloneFile clones the file from to the file to.
+// cloneFile makes a clone of a file via MacOS's `clonefile` syscall.
 func cloneFile(from, to string) error {
 	return unix.Clonefile(from, to, 0)
 }

clonefile_other.go

@@ -5,7 +5,9 @@ package testscript
 
 import "fmt"
 
-// We don't want to use hard links on Windows, as that can lead to "access denied" errors when removing.
+// cloneFile does not attempt anything on Windows, as hard links on it have
+// led to "access denied" errors when deleting files at the end of a test.
+// We haven't tested platforms like plan9 or wasm/wasi.
 func cloneFile(from, to string) error {
 	return fmt.Errorf("unavailable")
 }