CI: run ethereum tests on ubuntu-latest

ubuntu-20.04 ships with GLIBC-2.31, but the latest foundry
nightly (which these CI actions use) link against 2.33+ (perhaps foundry
should statically link against something like musl...?)

So instead we run these two tests on ubuntu-latest

.github/workflows/build.yml

@@ -63,7 +63,7 @@ jobs:
       - run: cd algorand && make test
 
   ethereum:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2
@@ -74,7 +74,7 @@ jobs:
       - run: cd ethereum && make test
 
   ethereum-upgrade:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2