ghc.mk: amend 'make sdist'

Noticed 'make sdist' failure seen as:

```
"rm" -rf sdistprep/ghc/ghc-9.1.0.20201020/hadrian/_build/ (SRC_DIST_GHC_DIR)/hadrian/dist-newstyle/
/bin/sh: -c: line 0: syntax error near unexpected token `('
```

commit 9657f6f3
("sdist: Include hadrian sources in source distribution")
added a new cleanup path without a variable expantion.

The change adds variable reference. While at it move directory
cleanup to a separate statement.

Amends #18794

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>