use the same build step as the actual release

[laverya]

specifically drop buildkite

What this PR does / why we need it:

The current build fails, because the buildkite image is really, really old.

This is the only place we use the buildkite image - including the prod release process, which the updated version is copied from.

kots/.github/workflows/release.yaml

Lines 88 to 106 in 8f9da3a

	build-web:
	runs-on: ubuntu-20.04
	needs: [generate-tag]
	steps:
	- name: Checkout
	uses: actions/checkout@v4
	- name: Setup Node.js environment
	uses: actions/setup-node@v3
	with:
	node-version: '17.x'
	- name: Build web
	env:
	GIT_TAG: ${{ needs.generate-tag.outputs.tag }}
	run: mapfile -t envs < <(grep -v '#.*' < .image.env) && export "${envs[@]}" && make -C web deps build-kotsadm
	- name: Upload web artifact
	uses: actions/upload-artifact@v3
	with:
	name: web
	path: ./web/dist

Looks a whole lot like actions/checkout#1474.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Steps to reproduce

Does this PR introduce a user-facing change?

Does this PR require documentation?