Stop building at local install time

This speeds up local development and CI runs by skipping build steps by default.
They will now only run when just before publishing.

Related to https://github.com/matrix-org/matrix-js-sdk/pull/1552

package.json

@@ -31,7 +31,7 @@
   "typings": "./lib/index.d.ts",
   "matrix_src_main": "./src/index.js",
   "scripts": {
-    "prepare": "yarn build",
+    "prepublishOnly": "yarn build",
     "i18n": "matrix-gen-i18n",
     "prunei18n": "matrix-prune-i18n",
     "diff-i18n": "cp src/i18n/strings/en_EN.json src/i18n/strings/en_EN_orig.json && ./scripts/gen-i18n.js && node scripts/compare-file.js src/i18n/strings/en_EN_orig.json src/i18n/strings/en_EN.json",