rapids-github-run-id: raise error if no run ID found

[jameslamb]

Fixes #192

Today, rapids-github-run-id can silently and "successfully" (exit code 0) return an empty string.

This is never desirable in RAPIDS CI... that empty string leads to artifact-downloading grabbing whatever the latests uploaded artifacts happened to be, which can lead to problems of the form "not building/testing against the packages I intended to build/test against".

This can be difficult to recognize if pulling the wrong artifact doesn't result in outright failures.

To avoid that, this PR proposes raising a big loud error in the situation where no run ID is found.

Notes for Reviewers

How I tested this

Fails in the expected way for a SHA that does not exist:

run_id=$(
    PATH="$(pwd)/tools:${PATH}" \
    RAPIDS_BUILD_TYPE=branch \
    RAPIDS_REPOSITORY="rapidsai/rmm" \
    RAPIDS_REF_NAME="branch-25.08" \
    RAPIDS_SHA="some-nonsense-that-does-not-exist" \
    RAPIDS_BUILD_WORKFLOW_NAME="build.yaml" \
      rapids-github-run-id
)

# [rapids-github-run-id] ERROR: failed to find a GitHub Actions run for [RAPIDS_BUILD_TYPE=branch, RAPID_REPOSITORY=rapidsai/rmm, RAPIDS_REF_NAME=branch-25.08, RAPIDS_SHA=some-nonsense-that-does-not-exist]

echo $?
# 1

Succeeds as expected for a real commit:

run_id=$(
    PATH="$(pwd)/tools:${PATH}" \
    RAPIDS_BUILD_TYPE=branch \
    RAPIDS_REPOSITORY="rapidsai/rmm" \
    RAPIDS_REF_NAME="branch-25.08" \
    RAPIDS_SHA="e5b1c01b8600e11872268484af3176a3eeeb8d03" \
    RAPIDS_BUILD_WORKFLOW_NAME="build.yaml" \
      rapids-github-run-id
)

echo $?
# 0

echo $run_id
# 15470752435

That exactly corresponds to https://github.com/rapidsai/rmm/actions/runs/15470752435/job/43554265779, which is correct.