Merge rust-bitcoin#489: ci: use ubuntu-slim for label & prepare ci tasks

f186642 ci: use ubuntu-slim for label & prepare ci tasks (0xb10c)

Pull request description:

  The `ubuntu-slim` 1vCPU runners were recently introduced and seem to be the right fit here.

  https://github.blog/changelog/2026-01-22-1-vcpu-linux-runner-now-generally-available-in-github-actions/

  https://docs.github.com/en/actions/reference/runners/github-hosted-runners#single-cpu-runners

ACKs for top commit:
  jamillambert:
    ACK f186642
  tcharding:
    ACK f186642

Tree-SHA512: c93b00b5eaa2b466ae004a75ddf0e72ee170d2f4da9174b0483faff5da48ed98f46ec91f80f31b33b2b78db3cf6b29a68f91b616621de8bbb42c5f57fe477028

Author: tcharding

.github/workflows/pr-labeler.yml

@@ -5,7 +5,7 @@ on:
 
 jobs:
   label-pr:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     permissions:
       contents: read
       pull-requests: write

.github/workflows/rust.yaml

@@ -10,7 +10,7 @@ name: Continuous integration
 
 jobs:
   Prepare:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     outputs:
       nightly_version: ${{ steps.read_toolchain.outputs.nightly_version }}
     steps: