Make set-max-old-space-size.sh compatible with sh

The . operator defaults to the current shell and
ignores the shebang so in non-bash shells, it is
not bash that will be used, which can result in
the following script failure:
```
> . scripts/set-max-old-space-size.sh && hardhat test

sh: 8: scripts/set-max-old-space-size.sh: [[: not found
```
The shebang could be respected by avoiding sourcing but
a better option is to simply remove the bash dependency
from the script since it is not required.

.changeset/wise-candles-rescue.md

@@ -0,0 +1,2 @@
+---
+---

scripts/set-max-old-space-size.sh

@@ -1,10 +1,10 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 # This script sets the node `--max-old-space-size` to 8192 if it is not set already.
 # All existing `NODE_OPTIONS` are retained as is.
 
 export NODE_OPTIONS="${NODE_OPTIONS:-}"
 
-if [[ $NODE_OPTIONS != *"--max-old-space-size"* ]]; then
+if [ "${NODE_OPTIONS##*--max-old-space-size*}" = "$NODE_OPTIONS" ]; then
   export NODE_OPTIONS="${NODE_OPTIONS} --max-old-space-size=8192"
 fi