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/fetchgit/nix-prefetch-git

@@ -256,11 +256,11 @@ else
   # download the file and add it to the store.
   if test -z "$finalPath"; then
 
-      tmpPath=/tmp/git-checkout-tmp-$$
-      tmpFile=$tmpPath/git-export
-      mkdir $tmpPath $tmpFile
+      tmpPath="$(mktemp --tmpdir -d git-checkout-tmp-XXXXXXXX)"
+      trap "rm -rf \"$tmpPath\"" EXIT
 
-      trap "rm -rf $tmpPath" EXIT
+      tmpFile="$tmpPath/git-export"
+      mkdir "$tmpFile"
 
       # Perform the checkout.
       clone_user_rev "$tmpFile" "$url" "$rev"