Fix PKG_BUILDNUM env variable for post-link scripts

[nsoranzo]

As documented in https://docs.conda.io/projects/conda-build/en/latest/user-guide/environment-variables.html , during the build process the PKG_BUILDNUM environment variable should contain the build number of the package being built, without the hash (which is instead part of the PKG_BUILD_STRING environment variable).

When calling the pre-link, post-link and pre-unlink scripts, PKG_BUILDNUM is one of the 4 environment variable that need to be set, see https://docs.conda.io/projects/conda-build/en/latest/resources/link-scripts.html (as is the case already), but here it was including also the hash. This breaks the installation of some bioconda packages (e.g. lotus2) when using micromamba (but not mamba, not sure why).

Fix the following error:

$ micromamba create --log-level trace -c conda-forge -c bioconda --channel-priority strict -n __lotus2@2.24 lotus2=2.24
...
debug    libmamba For lotus2 at /home/soranzon/micromamba/envs/__lotus2@2.24, executing script: $ /bin/bash /tmp/mambafWdlBXMccxn
trace    libmamba Calling /bin/bash /tmp/mambafWdlBXMccxn
trace    libmamba ENV MAP:
     ROOT_PREFIX: /home/soranzon/micromamba
     PREFIX: /home/soranzon/micromamba/envs/__lotus2@2.24
     PKG_NAME: lotus2
     PKG_VERSION: 2.24
     PKG_BUILDNUM: hdfd78af_0
     PATH: /home/soranzon/micromamba/envs/__lotus2@2.24/bin:/home/soranzon/bin:/home/soranzon/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
     CWD: /home/soranzon/micromamba/envs/__lotus2@2.24/bin
/home/soranzon/micromamba/envs/__lotus2@2.24/bin/.lotus2-post-link.sh: line 4: cd: /home/soranzon/micromamba/envs/__lotus2@2.24/share/lotus2-2.24-hdfd78af_0/: No such file or directory
Can't open perl script "autoInstall.pl": No such file or directory
...

where PKG_BUILDNUM should have been simply 0.
The relevant part of the post-link.sh file of the lotus2 recipe is:

TARGET=$PREFIX/share/$PKG_NAME-$PKG_VERSION-$PKG_BUILDNUM

cd "$TARGET/"
perl autoInstall.pl -condaDBinstall

The same package is installed fine by both conda and mamba, that correctly find and run the __lotus2@2.24/share/lotus2-2.24-0/autoInstall.pl script.

Found by @hildebra .

[wolfv]

Thanks!