build-support: Use mktemp -d in nix-prefetch-*.

Instead of relying on $$ to not collide with an existing path.

Quoting the Bash manual about $$:

> Expands to the process ID of the shell. In a () subshell, it expands
> to the process ID of the current shell, not the subshell.

So, this is different from $BASHPID:

> Expands to the process ID of the current bash process. This differs
> from $$ under certain circumstances, such as subshells that do not
> require bash to be re-initialized.

But even $BASHPID is prone to race conditions if the process IDs wrap
around, so to be on the safe side, we're using mktemp here.

Closes #3784.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/build-support/fetchhg/nix-prefetch-hg

@@ -35,11 +35,10 @@ fi
 # download the file and add it to the store.
 if test -z "$finalPath"; then
 
-    tmpPath=/tmp/hg-checkout-tmp-$$
-    tmpArchive=$tmpPath/hg-archive
-    mkdir $tmpPath
+    tmpPath="$(mktemp --tmpdir -d hg-checkout-tmp-XXXXXXXX)"
+    trap "rm -rf \"$tmpPath\"" EXIT
 
-    trap "rm -rf $tmpPath" EXIT
+    tmpArchive="$tmpPath/hg-archive"
 
     # Perform the checkout.
     if [[ $url != /* ]]; then