Run dist artifact builds on pull requests

[devfire]

What

Set pr-run-mode = "upload" in dist-workspace.toml so cargo-dist builds the full 6-target artifact matrix on PRs instead of only running dist plan.

Why

PR runs of the Release workflow previously skipped build-local-artifacts (and everything downstream) because publishing is false on pull_request events and the default pr_run_mode is plan. That meant target-specific build breakage (e.g. windows-msvc only) surfaced only after tagging a release. With upload, builds run on every PR and artifacts attach to the workflow run; host/announce still skip, so nothing is ever released from a PR.

Note: cargo-dist forces needs_coherence=true in this mode — workspace versions must match tags in lockstep; single-package workspace, so trivially satisfied.

Deviations

• The dist generate-ci regeneration also adds a swatinem/rust-cache@v2 step to build-local-artifacts (4 lines). Blocked from push by missing workflow token scope; follows as a second commit once scope is granted. The behavior change does not depend on it — the job if: reads pr_run_mode from the runtime plan manifest, which comes from this toml change.

Verification

• dist plan --output-format=json locally: ci.github.pr_run_mode == "upload", all 6 targets present in the artifacts matrix, exit 0.
• This PR's Release run is the end-to-end check: build jobs must start instead of skipping.