build: stop installing golangci-lint on the host

The linter runs only inside the pinned golangci-lint container image
(Dockerfile.lint, and the Dockerfile lint stage). Remove the host
install that was still left behind: script/bootstrap's version and
sha256 pins, the release download, the ensure/verify helpers, and the
curl dependency they were the only user of; and drop golangci-lint from
the nix-shell package lists in the Makefile and script/test, which build
and test but never lint.

Model: opus-4-8

script/test

@@ -10,7 +10,7 @@ run_with_cgo_deps() {
     if command -v pkg-config >/dev/null 2>&1; then
         sh -c "$1"
     else
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+        nix-shell -p pkg-config vips libheif git --run "$1"
     fi
 }