Pin down Rust toolchain (#1719)

Summary:
To make sure we're staying in sync with the internal version. Requires some more manual updating, but even if we do this every couple of months, it should be fine.

Pull Request resolved: https://github.com/facebook/flipper/pull/1719

Test Plan: CI

Reviewed By: nikoant

Differential Revision: D25195565

Pulled By: passy

fbshipit-source-id: 763a9d877a20659e91c8b77d0900b999e5f630d3

.github/workflows/packer.yml

@@ -11,7 +11,7 @@ jobs:
       - name: Setup toolchain
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: 1.48.0
       - name: Test
         run: cd packer && cargo test
       - name: Format