Add support to build flipper-server from SandCastle

Summary:
This refactors the flipper-server release script in such a way that it works the same as the normal release script, which solves two problems:

1) the official release script modifies versioned files, as it touches the package.json
2) it was slightly confusing that `flipper-server/static` was filled for release builds only, but not used in dev builds
3) running test:npx without running a release build before it, would fail with hard to comprehend errors. this has been solved now by removing that script and make it an arg of `build:flipper-server`

Also some further minor changes to prepare running a corresponding build / release job from SandCastle (later in this stack)

Reviewed By: nikoant

Differential Revision: D33297214

fbshipit-source-id: f6299aa982c3e59d1cc6479a93c56cbe4b57f85c

desktop/flipper-server-core/src/utils/environmentInfo.tsx

@@ -14,11 +14,11 @@ import path from 'path';
 import {EnvironmentInfo, ReleaseChannel} from 'flipper-common';
 
 export async function getEnvironmentInfo(
-  staticPath: string,
+  packageJsonDir: string,
   isProduction: boolean,
 ): Promise<EnvironmentInfo> {
   const packageJson = await fs.readJSON(
-    path.resolve(staticPath, 'package.json'),
+    path.resolve(packageJsonDir, 'package.json'),
   );
 
   const releaseChannel: ReleaseChannel =