Add /root/.pixi/bin to the PATH and associated test

[hadim]

Fix #40

This is fragile since it won't work if the user is modified, but at least it will make pixi global binaries available "by default" without any additional tweaks.

[hadim]

@pavelzw what do you think about pre-installing git with pixi global install given #41 ?

[pavelzw]

I think the size increase is too much when not all users benefit from it (git is only used in pypi dependencies and I personally still think that you should package everything as a conda package if possible).

❯ docker history pixi-docker-no-git
IMAGE          CREATED         CREATED BY                                      SIZE      COMMENT
9eb4291ea7ca   4 minutes ago   ENV PATH=/root/.pixi/bin:/usr/local/sbin:/us…   0B        buildkit.dockerfile.v0
<missing>      4 minutes ago   COPY --chown=root:root --chmod=0555 /pixi /u…   32.6MB    buildkit.dockerfile.v0
<missing>      2 weeks ago     # debian.sh --arch 'arm64' out/ 'bookworm' '…   97.2MB    debuerreotype 0.15
❯ docker history pixi-docker-git
IMAGE          CREATED              CREATED BY                                      SIZE      COMMENT
62947da63edb   About a minute ago   RUN /bin/sh -c pixi global install git && pi…   218MB     buildkit.dockerfile.v0
<missing>      About a minute ago   ENV PATH=/root/.pixi/bin:/usr/local/sbin:/us…   0B        buildkit.dockerfile.v0
<missing>      4 minutes ago        COPY --chown=root:root --chmod=0555 /pixi /u…   32.6MB    buildkit.dockerfile.v0
<missing>      2 weeks ago          # debian.sh --arch 'arm64' out/ 'bookworm' '…   97.2MB    debuerreotype 0.15

for reference: pixi-docker-git was created by adding RUN pixi global install git && pixi clean cache -y to the end of the dockerfile

If users need git, they can just global install it themselves (or do it via apt-get)