fix(makefile): hardcode era for test-env setup

The test-env Makefile target now always sets up the environment for the
'conway' era, removing the need to specify 'era=conway' as a parameter.
Updated the README to reflect this usage change for consistency and
clarity.

Author: mkoura

Makefile

@@ -20,11 +20,7 @@ reinstall-editable:
 # set up test environment
 .PHONY: test-env
 test-env:
-	@if [ -z "$(era)" ]; then \
-		echo "Usage: make test-env era=conway" >&2; \
-		exit 1; \
-	fi
-	@./scripts/setup_test_env.sh "$(era)"
+	@./scripts/setup_test_env.sh conway
 
 # initialize linters
 .PHONY: init_lint

README.md

@@ -80,7 +80,7 @@ For workflows requiring repeated test runs on a persistent testnet cluster:
 2. Set up the local test environment:
 
   ```sh
-  make test-env era=conway
+  make test-env
   ```
 
 3. Activate the environment: