all: update shebangs for more portability
NixOS by default doesn't ship with /bin/bash, rather /usr/bin/env bash is the way to get the right bash.
This caused two issues on NixOS:
1. The `push` button on the web interface would fail with ` ! [remote rejected] <commit> -> <branch> (pre-receive hook declined)`
2. `make` would result in an error
On the host side, for 1 you could see `fatal: cannot exec '/tmp/sketch-git-hooks-3993344797/pre-receive': No such file or directory`, and for 2, you could similarly see `make: ./build/webui.sh: No such file or directory`
This fixes both of those errors by updating to more portable shebangs.
```
diff --git a/loop/update_tests.sh b/loop/update_tests.sh
index 1e78edd..ab5e2e0 100755
--- a/loop/update_tests.sh
+++ b/loop/update_tests.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
CURRENT_DIR=$(pwd)