Bootstrap keeps yarn and node only at their pinned versions (closes #154)

script/bootstrap kept any yarn found on PATH, so YARN_VERSION applied
only where none was installed. It now keeps the yarn on PATH only when
yarn --version is exactly YARN_VERSION, and otherwise installs that
version as before. The node on PATH is used only when it has the
pinned major version, as REPO_POLICIES.md asks, and is at least
NODE_MIN_VERSION. After installing either, bootstrap looks the tool up
through PATH again, stops, naming the path, if that one fails the same
check, and prints the version it uses, as it does for Go. Every host
script except bootstrap that runs yarn now runs it through the new
script/yarn, which uses the first node on PATH, with ~/.local/bin
first, when it passes the same test, and otherwise the pinned node in
nvm.

Model: opus-5-5

script/build

@@ -8,7 +8,7 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    yarn build
+    "$ROOT/script/yarn" build
 
     # VERSION comes from the environment. Unset or empty, it is git
     # describe, or "dev" where there is no git or no repository history.