Fix admin router syntax for axum 0.8

[palfrey]

Description

Right now, if you run nix run -L .#nativelink -- ./nativelink-config/examples/basic_cas.json5 you get

thread 'main' panicked at src/bin/nativelink.rs:672:31:
Path segments must not start with `:`. For capture groups, use `{capture}`. If you meant to literally match a segment starting with a colon, call `without_v07_checks` on the router.

because #1674 upgraded axum to 0.8.x and they changed the router syntax, but there's no tests that run a config with the admin enabled. #1646 does do this, and hence a) why I've hit it and b) why I haven't added a test yet, as that PR will do the testing.

Type of change

Please delete options that aren't relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

nix run -L .#nativelink -- ./nativelink-config/examples/basic_cas.json5 from current main

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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This change is 

[aaronmondal]

Thanks!

+@jaroeichler

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: 1 of 2 LGTMs obtained, and all files reviewed, and pending CI: Analyze (javascript-typescript), Analyze (python), Bazel Dev / macos-15, Bazel Dev / ubuntu-24.04, Cargo Dev / macos-15, Cargo Dev / ubuntu-24.04, Coverage, Installation / macos-13, Installation / macos-14, Installation / macos-15, Installation / ubuntu-22.04, Installation / ubuntu-24.04, Local / lre-cc / ubuntu-24.04, Local / lre-rs / macos-15, Local / lre-rs / ubuntu-24.04, NativeLink.com Cloud / Remote Cache / macos-15, NativeLink.com Cloud / Remote Cache / ubuntu-24.04, Publish image, Publish nativelink-worker-init, Publish nativelink-worker-lre-cc, Remote / lre-cc / large-ubuntu-22.04, Remote / lre-rs / large-ubuntu-22.04, asan / ubuntu-24.04, integration-tests (24.04), macos-15, pre-commit-checks, ubuntu-24.04, ubuntu-24.04 / stable, windows-2022 / stable (waiting on @jaroeichler)

[jaroeichler]

Reviewed all commit messages.
Reviewable status: complete! 2 of 2 LGTMs obtained, and all files reviewed

[MarcusSorealheis]

BEAMers