build: use GIT_CACHE_PATH (#14165)

* build: add GIT_CACHE_PATH

The merge at 1105473 was to specify git cache entirely from the environment variable  [and] removal of the general --cache-dir option from gclient

* docs: document GIT_CACHE_PATH in GN build

* fix: try to fix appveyor-gn env syntax

docs/development/build-instructions-gn.md

@@ -29,6 +29,19 @@ try to download a Google-internal version that only Googlers have access to).
 
 ## Getting the Code
 
+### Using a Git cache (optional step)
+
+`gclient` fetches about 16G worth of repository data. If you plan on building
+more than once, consider using its cache feature to make future calls faster:
+
+```sh
+$ export GIT_CACHE_PATH="$HOME/.git_cache"
+$ mkdir -p "$GIT_CACHE_PATH"
+# This will take about 16G.
+```
+
+### Getting the code with gclient
+
 ```sh
 $ mkdir electron-gn && cd electron-gn
 $ gclient config \