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/stdenv/generic/setup.sh

@@ -495,7 +495,7 @@ dumpVars() {
 stripHash() {
     strippedName=$(basename $1);
     if echo "$strippedName" | grep -q '^[a-z0-9]\{32\}-'; then
-        strippedName=$(echo "$strippedName" | cut -c34-)
+        echo "$strippedName" | cut -c34-
     fi
 }