feat: add flow-binary input for testing unreleased builds

[jahvon]

Blocks flowexec/flow#446, which needs this to run its CI tasks with the flow binary under review.

Summary

Adds a flow-binary input so a caller can run its executables with a flow binary it just built, instead of one installed from a release or built from main.

- run: go build -o ./bin/flow .
- uses: flowexec/action@v1
  with:
    executable: 'test unit'
    flow-binary: ./bin/flow

Why

Without it, a repository whose CI runs flow tasks is always testing a flow that predates the code under review. That is a silent gap in general, and a hard block when a task depends on a flow feature that has not shipped yet.

It surfaced concretely in flowexec/flow#439, which adds Python execution. A .execs target running a .py script failed CI on every platform:

Executing: flow test python-script
unable to parse file - 14:1: `foo(` must be followed by `)`

flow-version: main clones flow from GitHub and builds main, so the branch adding .py support was discarded and the released flow parsed the Python file as shell. No change to that PR could have made the job pass — the capability has to be on main before CI can use it, which is circular.

Notable Changes

• flow-binary takes precedence over flow-version.
• The download cache step is skipped when it is set — there is nothing to restore.
• A missing path fails loudly rather than falling back to a download, so a typo cannot silently reintroduce the gap this exists to close.
• Handles the Windows install path ($HOME/bin/flow.exe) alongside the Unix one, matching the existing main-build branch.
• No behavior change when the input is unset.

Testing

The action has no CI workflows to add a case to. Verified:

• action.yaml parses as valid YAML; bash -n scripts/install-flow.sh is clean.
• The new branch is the first condition, so the cached / latest / main / pinned-version paths are untouched when flow-binary is empty.
• End-to-end verification is flowexec/flow#446, which consumes this input across its CI matrix (ubuntu, macos, windows). Worth landing this and moving the v1 tag before that PR goes green.

🤖 Generated with Claude Code

https://claude.ai/code/session_01R328pa3FUUfga4gYah1iQi