Use mtime to measure times - #70

✅ (analysis) (passed)
✅ (lint-doc) (passed)
✅ (lint-fmt) (passed)
✅ (lint-opam) (passed)
✅ (lower-bound) (passed)
✅ alpine-3.23-4.14_opam-2.5 (passed)
✅ alpine-3.23-5.5_opam-2.5 (passed)
✅ debian-12-4.14_opam-2.5 (passed)
✅ debian-12-5.5_opam-2.5 (passed)
✅ debian-13-4.11_opam-2.5 (passed)
✅ debian-13-4.14+flambda_opam-2.5 (passed)
✅ debian-13-4.14_arm64_opam-2.5 (passed)
✅ debian-13-4.14_opam-2.5 (passed)
✅ debian-13-4.14_ppc64_opam-2.5 (passed)
✅ debian-13-4.14_riscv64_opam-2.5 (passed)
✅ debian-13-4.14_s390x_opam-2.5 (passed)
✅ debian-13-5.2_opam-2.5 (passed)
✅ debian-13-5.4_opam-2.5 (passed)
✅ debian-13-5.5+flambda_opam-2.5 (passed)
✅ debian-13-5.5_arm64_opam-2.5 (passed)
✅ debian-13-5.5_opam-2.5 (passed)
✅ debian-13-5.5_ppc64_opam-2.5 (passed)
✅ debian-13-5.5_riscv64_opam-2.5 (passed)
✅ debian-13-5.5_s390x_opam-2.5 (passed)
✅ fedora-42-4.14_opam-2.5 (passed)
✅ fedora-42-5.5_opam-2.5 (passed)
✅ fedora-43-4.14_opam-2.5 (passed)
✅ fedora-43-5.5_opam-2.5 (passed)
✅ fedora-44-4.14_opam-2.5 (passed)
✅ fedora-44-5.5_opam-2.5 (passed)
❌ freebsd-15.1-4.14_opam-2.5 (failed: Cancelled)
❌ freebsd-15.1-5.5_opam-2.5 (failed: Cancelled)
✅ macos-homebrew-4.14_arm64_opam-2.5 (passed)
✅ macos-homebrew-4.14_opam-2.5 (passed)
✅ macos-homebrew-5.5_arm64_opam-2.5 (passed)
✅ macos-homebrew-5.5_opam-2.5 (passed)
✅ openbsd-79-amd64-4.14_opam-2.5 (passed)
✅ openbsd-79-amd64-5.5_opam-2.5 (passed)
✅ opensuse-16.0-4.14_opam-2.5 (passed)
✅ opensuse-16.0-5.5_opam-2.5 (passed)
✅ ubuntu-22.04-4.14_opam-2.5 (passed)
✅ ubuntu-22.04-5.5_opam-2.5 (passed)
✅ ubuntu-24.04-4.14_opam-2.5 (passed)
✅ ubuntu-24.04-5.5_opam-2.5 (passed)
✅ ubuntu-25.04-4.14_opam-2.5 (passed)
✅ ubuntu-25.04-5.5_opam-2.5 (passed)
✅ ubuntu-25.10-4.14_opam-2.5 (passed)
✅ ubuntu-25.10-5.5_opam-2.5 (passed)
✅ ubuntu-26.04-4.14_opam-2.5 (passed)
✅ ubuntu-26.04-5.5_opam-2.5 (passed)
❌ EXPERIMENTAL: windows-server-mingw-ltsc2025-5.5_opam-2.5 (failure: Failed: Build failed)