ci: fix nightly build after etcd action update (#2208)

The regression is introduced in commit 01e0d20 ("Use common etcd
action instead of local (#2196)"). The fix follows a way how another
workflow is fixed in commit c4566cd ("ci: fix etcd tests in
reusable-backend-test.yml (#2202)").

The problem is that the new setup-etcd action sets the `ETCD_PATH`
environment variable on its own. The action sets it as a path to the
directory with the `etcd` executable. However, cartridge testing expects
a path to the executable itself in this variable.

The workflow-level environment variable definition is rewritten by the
action. The fix is to move the environment variable definition down to
the job step level to overwrite action's `ETCD_PATH`.

Reported-by: Yaroslav Dynnikov <[email protected]>

.github/workflows/nightly-test.yml

@@ -8,8 +8,6 @@ on:
 env:
   # Skip building frontend in tarantoolctl rocks make
   CMAKE_DUMMY_WEBUI: true
-  # Prerequisite for some etcd-related tests
-  ETCD_PATH: etcd-v2.3.8/etcd
 
 jobs:
   run-tests:
@@ -50,6 +48,9 @@ jobs:
         run: sudo kill -9 $(sudo lsof -t -i tcp:8084) || true
 
       - run: .rocks/bin/luatest -v
+        env:
+          # Prerequisite for some etcd-related tests
+          ETCD_PATH: etcd-v2.3.8/etcd
 
       # Cleanup cached paths
       - run: tarantoolctl rocks remove cartridge