stdenv/stripHash: print to stdout, not to variable

`stripHash` documentation states that it prints out the stripped name to
the stdout, but the function stored the value in `strippedName`
instead.

Basically all usages did something like
`$(stripHash $foo | echo $strippedName)` which is just braindamaged.
Fixed the implementation and all invocations.

pkgs/build-support/vm/default.nix

@@ -537,8 +537,7 @@ rec {
 
       # Hacky: RPM looks for <basename>.spec inside the tarball, so
       # strip off the hash.
-      stripHash "$src"
-      srcName="$strippedName"
+      srcName="$(stripHash "$src")"
       cp "$src" "$srcName" # `ln' doesn't work always work: RPM requires that the file is owned by root
 
       export HOME=/tmp/home