Delete `build` folder after compilation

[Aghassi]

• NPM version (npm -v): 6.14.15
• Node version (node -v): 14.18.0
• Node Process (node -p process.versions):

{
  node: '14.18.0',
  v8: '8.4.371.23-node.84',
  uv: '1.42.0',
  zlib: '1.2.11',
  brotli: '1.0.9',
  ares: '1.17.2',
  modules: '83',
  nghttp2: '1.42.0',
  napi: '8',
  llhttp: '2.1.3',
  openssl: '1.1.1l',
  cldr: '39.0',
  icu: '69.1',
  tz: '2021a',
  unicode: '13.0'
}

• Node Platform (node -p process.platform): darwin
• Node architecture (node -p process.arch): x64
• node-sass version (node -p "require('node-sass').info"):

node-sass	6.0.0	(Wrapper)	[JavaScript]
libsass  	3.5.5	(Sass Compiler)	[C/C++]

• npm node-sass versions (npm ls node-sass): 6.0.0

The bug is a minor one, but when resolving node-sass through actions under Bazel, the build folder that is created during the post install process has files with SHAs in them. That means that installs are not a cache hit because it changes between resolutions. The fix for this is just to have node-sass cleanup after itself on postinstall such that it removes the build folder when done building and copying files around.