Remove support for environment variables

[hiker]

While supporting environment variables is a valid suggestion (for backwards compatibility with UKMO builds), imho Fab itself should NOT use environment variables to define compiler, linker, or flags.

First of all, this can cause test failures (if environment variables add flags to compiler calls). Second of all, it can be very counterintuitive and hard to debug if Fab is used for other software.

Instead, the scripting layer on top of Fab can (If required) support this. We already do this in our LFRic builds, our command-line tool (where you can define compiler etc) will default to the corresponding environment variable, and define the corresponding compiler.

Similarly, if required, the zero-config command line interface to Fab could query the variables and trigger the corresponding settings (if we really desire that).

[hiker]

We agreed to remove the support for this, and move the support for this into the command line interface (for zero config usages)