ENH: Add support for linux/aarch64; simplify fslversion

The fslversion script had some flaky logic to try and parse the commands in the conda environment revision history, so it could print e.g. "Install FSL 6.0.7.18". This has been removed, and now the raw commands are printed (the only sanitisation step being replacing the full /path/to/fsl/ with literal $FSLDIR).