Make it easier to just rebuild the WASM files (#3510)

* Adds `WASM_TARGET_DIRECTORY` env variable to `wasm-builder`

* Create the `wasm-builder-runner`'s in a common workspace

* Make `wasm-builder` trigger less rebuilds

* Version up

* Adds script for building only the WASM files

* Apply suggestions from code review

Co-Authored-By: André Silva <andre.beat@gmail.com>
Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

substrate/README.adoc

@@ -321,6 +321,8 @@ we support multiple environment variables:
 * `TRIGGER_WASM_BUILD` - Can be set to trigger a WASM build. On subsequent calls the value of the variable
                          needs to change. As WASM builder instructs `cargo` to watch for file changes
                          this environment variable should only be required in certain circumstances.
+* `WASM_TARGET_DIRECTORY` - Will copy any build WASM binary to the given directory. The path needs
+                            to be absolute.
 
 Each project can be skipped individually by using the environment variable `SKIP_PROJECT_NAME_WASM_BUILD`.
 Where `PROJECT_NAME` needs to be replaced by the name of the cargo project, e.g. `node-runtime` will