fix(gatsby-adapter-netlify): handler generation on windows

[pieh]

Description

This fixes quite a bit of windows specific issues:

• (core) API functions compilation problems (path manipulation for require args had windows paths which can't be used)
• (core) handler produced on non-linux platform was unusable when deployed due to LMDB and sharp binaries - this allow to "cross-compile" and deploy from windows to linux - new CLI toggles and env vars were added to control target, but by default when using Netlify adapter we will target linux/x64. There will be meaningful error message shown when incompatible binaries were used mentioning toggles to use
• (adapter) function wrapper had windows paths in require args which can't be used

Tests

Smoke test building and deploying our adapters e2e test site on Windows and doing very brief/smoke test (can we hit index/SSG page, can we hit SSR page and get successful render was added)