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/scripts/build-only-wasm.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env sh
+
+# Script for building only the WASM binary of the given project.
+
+set -e
+
+PROJECT_ROOT=`git rev-parse --show-toplevel`
+
+if [ "$#" -lt 1 ]; then
+  echo "You need to pass the name of the crate you want to compile!"
+  exit 1
+fi
+
+WASM_BUILDER_RUNNER="$PROJECT_ROOT/target/release/wbuild-runner/$1"
+
+if [ -z "$2" ]; then
+  export WASM_TARGET_DIRECTORY=$(pwd)
+else
+  export WASM_TARGET_DIRECTORY=$2
+fi
+
+if [ -d $WASM_BUILDER_RUNNER ]; then
+  export DEBUG=false
+  export OUT_DIR="$PROJECT_ROOT/target/release/build"
+  cargo run --release --manifest-path="$WASM_BUILDER_RUNNER/Cargo.toml" \
+    | grep -vE "cargo:rerun-if-|Executing build command"                  \
+else
+  cargo build --release -p $1
+fi