chore: unify build and build PRs workflow (#5570)

## About the changes
Our build and build_prs workflows are almost the same with some minor
details:

1. **Build** also builds the frontend, but **PR Build** does not by
specifying `--ignore-scripts`
1. **Build PR** uploads test results: Upload test report to build. We'll
ignore this when building on main
1. **Build PR** just builds the backend (reason why the build is much
faster: 50s vs ~5m). We have a workflow for building frontend PRs, we'd
be removing the build frontend from the build on main, but we're already
exercising that to upload to dockerhub:

![image](https://github.com/Unleash/unleash/assets/455064/36f5a892-24dc-4de2-91bd-8e3419757b0d)
 
These are the main differences:

![image](https://github.com/Unleash/unleash/assets/455064/837b1ea4-305d-4472-b903-aeed7f3f3e7f)

This PR unifies the two workflows into one

.github/workflows/build.yaml

@@ -1,4 +1,4 @@
-name: Build & Tests
+name: Build, Lint & Tests
 
 on:
   push:
@@ -7,7 +7,11 @@ on:
     paths-ignore:
       - website/**
       - coverage/**
-
+  pull_request:
+    paths-ignore:
+      - frontend/**
+      - website/**
+      - coverage/**
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -39,11 +43,17 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
           cache: 'yarn'
-      - run: yarn
-      - run: yarn build:frontend:if-needed
+      - run: yarn install --frozen-lockfile --ignore-scripts
       - run: yarn lint
+      - run: yarn build:backend
       - run: yarn run test:report # This adds test results as github check to the workflow
         env:
           CI: true
           TEST_DATABASE_URL: postgres://postgres:postgres@localhost:5432/postgres
           DATABASE_URL: postgres://postgres:postgres@localhost:5432/postgres
+      - name: Upload test report to build # Done this way since external PRs would not be able to write the check. See https://github.com/marketplace/actions/test-reporter#recommended-setup-for-public-repositories
+        uses: actions/upload-artifact@v3
+        if: (success() || failure()) && github.ref == 'refs/heads/main'
+        with:
+          name: test-results
+          path: ./reports/jest-junit.xml