Update coverage report to codecov (#606)

Coveralls has had multiple stability issues in the recent while, and codecov seems much better maintained in general.

.github/codecov.yml

@@ -0,0 +1 @@
+comment: false

.github/workflows/build.yml

@@ -76,20 +76,10 @@ jobs:
     - name: make coverage
       run: |
         CXX=clang++-10 make -j2 config=coverage coverage
-    - name: debug coverage
-      run: |
-        git status
-        git log -5
-        echo SHA: $GITHUB_SHA
     - name: upload coverage
-      uses: coverallsapp/github-action@master
+      uses: codecov/codecov-action@v3
       with:
-        path-to-lcov: ./coverage.info
-        github-token: ${{ secrets.GITHUB_TOKEN }}
-    - uses: actions/upload-artifact@v2
-      with:
-        name: coverage
-        path: coverage
+        files: ./coverage.info
 
   web:
     runs-on: ubuntu-latest

README.md

@@ -1,4 +1,4 @@
-Luau ![CI](https://github.com/Roblox/luau/workflows/build/badge.svg) [![Coverage](https://coveralls.io/repos/github/Roblox/luau/badge.svg?branch=master&t=2PXMow)](https://coveralls.io/github/Roblox/luau?branch=master)
+Luau ![CI](https://github.com/Roblox/luau/workflows/build/badge.svg) [![codecov](https://codecov.io/gh/Roblox/luau/branch/master/graph/badge.svg?token=S3U44WN416)](https://codecov.io/gh/Roblox/luau)
 ====
 
 Luau (lowercase u, /ˈlu.aʊ/) is a fast, small, safe, gradually typed embeddable scripting language derived from [Lua](https://lua.org).