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/build/clean.sh b/build/clean.sh
index d6117da..2391f6a 100755
--- a/build/clean.sh
+++ b/build/clean.sh
@@ -1,6 +1,6 @@
-#!/bin/bash
+#!/usr/bin/env bash
set -e
rm -f sketch
rm -rf embedded/sketch-linux embedded/webui-dist
-rm -rf webui/node_modules webui/dist
\ No newline at end of file
+rm -rf webui/node_modules webui/dist