Update workflow configurations to use ubuntu-latest (#21)

Author: jmjoy

.github/workflows/license.yml

@@ -22,7 +22,7 @@ on:
 
 jobs:
  license:
- runs-on: ubuntu-20.04
+ runs-on: ubuntu-latest
  steps:
  - uses: actions/checkout@v2
  with:

.github/workflows/publish.yml

@@ -29,7 +29,7 @@ jobs:
  publish:
  name: Publish
 
- runs-on: ubuntu-20.04
+ runs-on: ubuntu-latest
  steps:
  - name: Checkout
  uses: actions/checkout@v2

.github/workflows/rust.yml

@@ -27,7 +27,7 @@ env:
 
 jobs:
  rust:
- runs-on: ubuntu-20.04
+ runs-on: ubuntu-latest
  steps:
  - name: Checkout
  uses: actions/checkout@v2