github: use ubuntu-20.04 instead of ubuntu-latest

GitHub is currently warning if we use ubuntu-latest. It is probably
better to be explicit anyway.

ev3dev-stretch needs to build it's own mpy-cross now since the Debian
Stretch docker image can't run a binary built on Ubuntu 20.04.

Author: dlech

.github/workflows/build.yml

@@ -14,7 +14,7 @@ env:
 jobs:
   mpy_cross:
     name: mpy-cross
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Checkout repo
       uses: actions/checkout@v2
@@ -32,7 +32,7 @@ jobs:
   unix_coverage:
     name: unix coverage
     needs: mpy_cross
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       PYBRICKS_NO_REDIRECT_STDOUT: 1
     steps:
@@ -65,8 +65,7 @@ jobs:
 
   ev3dev_stretch:
     name: ev3dev-stretch
-    needs: mpy_cross
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Docker login
       uses: azure/docker-login@v1
@@ -83,17 +82,12 @@ jobs:
       with:
         submodules: true
         fetch-depth: 0
-    - name: Download mpy-cross
-      uses: actions/download-artifact@v1
-      with:
-        name: mpy-cross
-        path: micropython/mpy-cross
-    - name: Fix file permission
-      run: chmod +x micropython/mpy-cross/mpy-cross
     - name: Create docker container
       run: bricks/ev3dev/docker/setup.sh armel
     - name: Build
-      run: docker exec --tty pybricks-ev3dev_armel make
+      run: |
+        docker exec --tty pybricks-ev3dev_armel make -C ../../micropython/mpy-cross CROSS_COMPILE=
+        docker exec --tty pybricks-ev3dev_armel make
     - name: Test
       if: ${{ success() }}
       run: |
@@ -112,7 +106,7 @@ jobs:
   ev3dev_bionic:
     name: ev3dev-bionic
     needs: mpy_cross
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CROSS_COMPILE: ""
     steps:
@@ -157,7 +151,7 @@ jobs:
 
   cross_compiler:
     name: download cross-compiler
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Check cache
       id: check-cache
@@ -179,7 +173,7 @@ jobs:
       matrix:
         hub: [cityhub, movehub, nxt, primehub, technichub]
     needs: [cross_compiler, mpy_cross]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Install cross-compiler
       uses: actions/cache@v1
@@ -242,7 +236,7 @@ jobs:
   debug:
     name: debug firmware
     needs: [cross_compiler, mpy_cross]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Install cross-compiler
       uses: actions/cache@v1

.github/workflows/format.yml

@@ -25,7 +25,7 @@ on:
 jobs:
   format:
     name: Check code formatting
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Install uncrustify
       run: |

.github/workflows/npm-firmware.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   npm_firmware:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     defaults:
       run:
         working-directory: npm/firmware

.github/workflows/npm-mpy-cross.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   npm_mpy_cross:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     defaults:
       run:
         working-directory: npm/mpy-cross

.github/workflows/release.yml

@@ -11,7 +11,7 @@ env:
 jobs:
   upload_release:
     name: Upload Release Assets
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Check cache
         id: check-cache