Revert --key flag, rebar3_hex reads HEX_API_KEY from env directly

The --key flag is not a valid option for rebar3 hex publish. The
rebar3_hex_config module reads HEX_API_KEY from the environment
via os:getenv and uses it directly as the API key. The env var
just needs to be set, no flag needed.

https://claude.ai/code/session_01RBbLduNgvjWctN2Dj12UDY

Author: claude

.github/workflows/publish.yml

@@ -61,14 +61,14 @@ jobs:
           restore-keys: ${{ runner.os }}-otp-27-build-
 
       - name: Publish to Hex.pm
-        run: rebar3 hex publish --yes --key "$HEX_API_KEY"
+        run: rebar3 hex publish --yes
         env:
           HEX_API_KEY: ${{ secrets.HEX_API_KEY }}
 
       - name: Generate docs
         run: rebar3 ex_doc
 
       - name: Publish docs
-        run: rebar3 hex docs publish --yes --key "$HEX_API_KEY"
+        run: rebar3 hex docs publish --yes
         env:
           HEX_API_KEY: ${{ secrets.HEX_API_KEY }}