ci: make musl.sh look for patches next to the script

[lambdageek]

Instead of hardcoding the container layout assuming that the local patches are in /build, look for them next to the script. This helps downstream distros to run this script if their container layout is different and they don't put the musl working dir in /build.

This might be an unexpected change for some downstream distros that were using this script from another directory,but arranged for the patches to still be in /build

[rustbot]

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[Mark-Simulacrum]

Hm, can you say a bit more about what your(?) distro is doing here? I would have somewhat expected that most / all distros want to use their own musl rather than whatever Rust happened to package (we're not necessarily amazing at staying up to date...).

If I'm reading the code right it seems pretty harmless to land this though.

[lambdageek]

Hm, can you say a bit more about what your(?) distro is doing here?

Context: we distribute in-house builds of the Rust toolchain for teams inside Microsoft (see blog - although that post is mostly talking about the special things we're doing on Windows, but we do also build Linux host tools and target several platforms using the normal cg_llvm backend)

For some targets, we build musl. When we build musl, we're building it in a step in an Azure Pipelines job, not in a Docker container like the ones upstream has defined, so we don't have an analog of these COPY steps:

rust/src/ci/docker/host-x86_64/dist-various-1/Dockerfile

Lines 67 to 69 in 88638df

	COPY scripts/musl.sh /build
	COPY scripts/musl-cve-2026-6042.diff /build/
	COPY scripts/musl-cve-2026-40200.diff /build/

We don't have a /build directory. I think the script and patches for us are in something like /s/.../src/ci/docker/scripts/musl.sh. So because musl.sh currently hardcodes /build/ to find those .patch files, we either need to make our environment more like upstream's (possible), or make the script less sensitive find the patches in a way where it doesn't matter where they all are as long as its all the same directory (preferred).

I would have somewhat expected that most / all distros want to use their own musl rather than whatever Rust happened to package (we're not necessarily amazing at staying up to date...).

The paved path for our internal Linux customers steers them to x86_64-unknown-linux-gnu (AzureLinux 3 and 4). There's just not that many who need musl, and we've not had a reason to get out of sync from upstream Rust packaging here.