Do not pin core dependencies

Core dependencies (boost, blas [see also #6 (closed)], python, etc) should not be pinned. The original reason for pinning them was to ensure that the same versions of all packages were the same at both build time and run time, but pinning them in the fsl-base package makes it very difficult to e.g. upgrade python, or switch the BLAS implementation, in an installed environment.

These packages could be instead pinned in the fsl-release.yml file at the fsl/conda/manifest> repository. To ensure consistency at both build and run time, the package build logic at fsl/conda/fsl-ci-rules> could scrape fsl-release.yml and install the core dependency versinos that are listed within.