feat: adding env var for "dirname" to mirror the logic in electron-builder

[mmaietta]

Current logic already uses env var ELECTRON_BUILDER_BINARIES_MIRROR

app-builder/pkg/download/tool.go

Lines 61 to 81 in 580c34f

	func downloadFromGithub(name string, version string, checksum string) (string, error) {
	id := name + "-" + version
	return DownloadArtifact(id, GetGithubBaseUrl()+id+"/"+id+".7z", checksum)
	}
	func GetGithubBaseUrl() string {
	v := os.Getenv("NPM_CONFIG_ELECTRON_BUILDER_BINARIES_MIRROR")
	if len(v) == 0 {
	v = os.Getenv("npm_config_electron_builder_binaries_mirror")
	}
	if len(v) == 0 {
	v = os.Getenv("npm_package_config_electron_builder_binaries_mirror")
	}
	if len(v) == 0 {
	v = os.Getenv("ELECTRON_BUILDER_BINARIES_MIRROR")
	}
	if len(v) == 0 {
	v = "https://github.com/electron-userland/electron-builder-binaries/releases/download/"
	}
	return v
	}

But it does not allow customization of the dir (both being the id of the asset)

app-builder/pkg/download/tool.go

Line 63 in 580c34f

return DownloadArtifact(id, GetGithubBaseUrl()+id+"/"+id+".7z", checksum)

This implementation now will properly mirror the (duplicated) logic in electron-builder for app-builder-bin downloading
https://github.com/electron-userland/electron-builder/blob/3d65267a6c53ca824f70e5b0f5d8f4ba8be38237/packages/app-builder-lib/src/binDownload.ts#L25-L38