Support BUILD_TOP environment for sandboxed builds

[poelzi]

Description

The build.rs assumes the source directory is writable, which breaks sandboxed builds like nix. Support BUILD_TOP environment variable, so build systems can simply set it to their build directory.

Checklist:

• Changelog

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[pronebird]

Should it not be NIX_BUILD_TOP according to https://nixos.wiki/wiki/Environment_variables?

[poelzi]

@pronebird yes, for nix specific builds yes. I thought it might be better to make this generic. I can change this, which would simplify the nix pkgs

[pronebird]

@pronebird yes, for nix specific builds yes. I thought it might be better to make this generic. I can change this, which would simplify the nix pkgs

This can be made generic for sure. I had impression that you've tested this with nix.

However note that build-wireguard-go.sh outputs artifacts in the very same directory, so this patch alone is not sufficient to make it all work.

Further thoughts: @trojanfoe we could probably stop touching file system from here and remove the call to canonicalize which is the reason for build failure when build dir does not exist.

[pronebird]

Let's merge it as is. Nice first step 👍 Thanks for contribution! 🚀