build(nix): stop shipping 727MB of rust-docs in every CI job's closure

[daniel-noland]

Stacked on #1746 — review/merge that first.

fromRustupToolchain applies components as extensions on top of a profile,
and profile defaults to rustup's default, which already carries rust-docs.
So rust-docs was arriving twice, and deleting it from the component list alone
would have removed nothing.

Naming profile = "minimal" is what actually drops it. Verified by set-diffing
the joined component paths of the toolchain derivation across three variants:

variant	rust-docs in the joined paths
today ("rust-docs" listed, no profile)	yes, twice
component deleted, profile still unset	yes, still there
this PR (deleted + profile = "minimal")	no

A full diff of the deduplicated path sets between the last two shows exactly
one line: rust-docs. Nothing else about the toolchain moves.
passthru.availableComponents.rust-src — which default.nix uses in three
places — still resolves to the same store path.

Why bother

727MB of prebuilt HTML, in the closure of all 8 dev-shell jobs. The lab runners
are ephemeral (directory /nix does not exist; creating it every job), so it is
pulled over the network each time, not cached on disk. Measured against the
current dev shell: 5.96GB closure, 325 paths, ~22s to realize.

Nothing in the tree reads it — grep for rust-docs, share/doc/rust, and
rustup doc finds only this comment.

Cost

Every rust-built store path in the shell changes hash, so the first CI run after
this merges repopulates that half of Cachix.

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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