ci: drop the nx cache that nx 22 cannot use

[mfal]

The nx cache restored by prepare-workspace never produced a single hit. Nx 20
replaced the file-system task cache with a database keyed by the machine ID —
/etc/machine-id on Linux, which every hosted runner generates fresh. The
restored .nx/cache is therefore artifacts nx has no metadata for, so it warns
and re-runs every task:

Cache hit for restore-key: nx-Linux-85bbc3b2d353a6a9b55348f45571ac6ff926a84a
NX  Unrecognized Cache Artifacts
Nx found unrecognized artifacts in the cache directory and will not be able to use them.
Nx can only restore artifacts it has metadata about.

Measured on run 33385191014: both preview build jobs spent 4.3s downloading and unpacking 15MB, then executed all of their tasks — 13 for docs, 11 for storybook — with zero cache hits.

Adding .nx/workspace-data to the cached paths does not fix it. The database
file is named after the machine ID, so nx never opens a restored one:

IOPlatformUUID = "2B7F7181-01D5-53E4-9886-E9280B9C5C73"
.nx/workspace-data/2B7F7181-01D5-53E4-9886-E9280B9C5C73-v2.db

The old escape hatches are gone as well: DbCache does not read
NX_REJECT_UNKNOWN_LOCAL_CACHE (only the deprecated Cache class did), and
useLegacyCache exists neither in nx 22's code nor in nx-schema.json.

What changes

• prepare-workspace: the restore step and its nx-cache input are gone, replaced by a comment recording why a local nx cache cannot work on CI — so the next reader does not re-add it
• test.yml: the main-only save step is gone, as is the now-pointless nx-cache: "false" on the lint job

Effect

Roughly 4–5s per job, across every job that uses prepare-workspace. Small — the
point is mainly that the previous comment promised something that never happened
("a hit turns those ~40s into unpacking"). Actually sharing build results across
runners needs a remote cache (Nx Cloud, or nx's self-hosted cache API); that is a
separate decision, not part of this PR.

Tooling-only, so this publishes nothing.

🤖 Generated with Claude Code

[github-actions]

Coverage Report for ./packages/components/

Status	Category	Percentage	Covered / Total
🔵	Lines	78.69%	746 / 948
🔵	Statements	78.57%	763 / 971
🔵	Functions	80.09%	165 / 206
🔵	Branches	70.33%	377 / 536

File Coverage

No changed files found.

Generated in workflow #6569 for commit 8031512 by the Vitest Coverage Report Action

[github-actions]

🚀 Preview Deployment

Preview environments are ready:

Type	URL
docs	pr-3034.docs.review.flow-components.de
storybook	pr-3034.storybook.review.flow-components.de

Images:

• docs: ghcr.io/mittwald/flow/docs:pr-3034
• storybook: ghcr.io/mittwald/flow/storybook:pr-3034