bug(build): pixi-build-cmake dep sets incorrect source dir with relative `source.path`

[lucascolley]

Here is the gist of the problem:

# packages/tests/pixi.toml
[package]
build = {
  backend.name = "pixi-build-cmake",
  source.path = "../..",
}

# pixi.toml
[dependencies]
xsf.path = "packages/tests"

Running pixi install in the root workspace yields the following build error:

CMake Error: The source directory "$PIXI_WORKSPACE_ROOT/packages/tests" does not appear to contain CMakeLists.txt.

The source directory is incorrectly set, it should be $PIXI_WORKSPACE_ROOT/packages/tests/../...

[lucascolley]

if this is fixed, I'd be very close to introducing pixi-build-cmake at scipy/xsf#202

[lucascolley]

Hmm I can't reproduce this any more, must have been due to something in the lock file or cache.