CI: rework and merge lint-openapi-spec and validate-compute-manifest jobs (#12575)

## Problem

We have several linters that use Node.js, but they are currently set up
differently, both locally and on CI.

## Summary of changes
- Add Node.js to `build-tools` image
- Move `compute/package.json` -> `build-tools/package.json` and add
`redocly` to it `@redocly/cli`
- Unify and merge into one job `lint-openapi-spec` and
`validate-compute-manifest`

compute/Makefile

@@ -50,9 +50,9 @@ jsonnetfmt-format:
 	jsonnetfmt --in-place $(jsonnet_files)
 
 .PHONY: manifest-schema-validation
-manifest-schema-validation: node_modules
-	node_modules/.bin/jsonschema validate -d https://json-schema.org/draft/2020-12/schema manifest.schema.json manifest.yaml
+manifest-schema-validation: ../build-tools/node_modules
+	npx --prefix=../build-tools/ jsonschema validate -d https://json-schema.org/draft/2020-12/schema manifest.schema.json manifest.yaml
 
-node_modules: package.json
-	npm install
-	touch node_modules
+../build-tools/node_modules: ../build-tools/package.json
+	cd ../build-tools && $(if $(CI),npm ci,npm install)
+	touch ../build-tools/node_modules

compute/package.json

@@ -1,7 +0,0 @@
-{
-  "name": "neon-compute",
-  "private": true,
-  "dependencies": {
-    "@sourcemeta/jsonschema": "9.3.4"
-  }
-}