CI improvements (#230)

- llvm artifacts search, download and extract now in `get-llvm` action 
- `get-emsdk` action - clone, install, activate emsdk, so we don't need
to build `revive-builder` for this
- switch workflows to new actions
- `concurrency` for remaining workflows (cancel run if new run is
triggered)
- `target_commitish` for main release, fixes release commit 
- Run release workflow in PR without creating a release if PR is labeled
with `release-test`

---------

Co-authored-by: cornholio <0@mcornholio.ru>
Co-authored-by: Alexander Theißen <alex.theissen@me.com>
Co-authored-by: xermicus <cyrill@parity.io>
Co-authored-by: xermicus <bigcyrill@hotmail.com>

.github/actions/get-emsdk/action.yml

@@ -0,0 +1,20 @@
+name: "get emsdk"
+inputs:
+  version:
+    description: ""
+    required: false
+    default: "3.1.64"
+
+
+runs:
+  using: "composite"
+  steps:
+
+    - name: install emsdk
+      shell: bash
+      run: |
+        git clone https://github.com/emscripten-core/emsdk.git ./emsdk/
+        cd emsdk
+        git checkout tags/${{ inputs.version }}
+        ./emsdk install ${{ inputs.version }}
+        ./emsdk activate ${{ inputs.version }}