Changed CI to just run on ubuntu-latest

If we already have to bump this version as GitHub phases out older
Ubuntu runners (which is reasonable), I don't really see the value of
pinning a specific version. We might as well just respond to any
broken dependencies caused by GitHub's implicit updates as they happen.

It's not like CI is truly continuous.

.github/workflows/test.yml

@@ -7,7 +7,7 @@ env:
 
 jobs:
   test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - uses: dtolnay/rust-toolchain@nightly
@@ -16,7 +16,7 @@ jobs:
         make test
 
   test-docs:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - uses: dtolnay/rust-toolchain@nightly
@@ -25,7 +25,7 @@ jobs:
         make docs
 
   test-configs:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - uses: dtolnay/rust-toolchain@nightly
@@ -34,7 +34,7 @@ jobs:
         make test-configs
 
   test-bench:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - uses: dtolnay/rust-toolchain@nightly