Add common crate to access env variables

In the future, we should use a proper secret store (not just env variables).
This lets us update one block of code and not n in the future.

.github/workflows/tests.yml

@@ -23,7 +23,8 @@ jobs:
           GITHUB_CI=true cargo test --all-features \
             -p std-shims \
             -p zalloc \
-            -p serai-db
+            -p serai-db \
+            -p serai-env
 
   test-crypto:
     runs-on: ubuntu-latest