Raise error when using BUILD_FOR envvars on multi-arch and unknown builds

[mr-cal]

What needs to get done

#4313 added the following environment variables for core20 snaps:

• SNAPCRAFT_ARCH_BUILD_ON
• SNAPCRAFT_ARCH_TRIPLET_BUILD_ON
• SNAPCRAFT_ARCH_BUILD_FOR
• SNAPCRAFT_ARCH_TRIPLET_BUILD_FOR

Multi-arch builds and builds with unknown architectures do not evaluate SNAPCRAFT_ARCH_BUILD_FOR or SNAPCRAFT_ARCH_TRIPLET_BUILD_FOR.

When these envvars are used in a snapcraft.yaml, an error should be raised, explaining to the user why they cannot use these envvars.

Why it needs to get done

This will improve the user experience for core20 snaps, where users are allowed to build for multiple architectures and unknown architectures.

[sergiusens]

In snapcraft.yaml

environment:
    LD_LIBRARY_PATH: /usr/lib/$SNAPCRAFT_ARCH_TRIPLET_BUILD_FOR

Would fail if multiple build fors exist, this could also fail correctly

override-build: |
     mv lib.a $SNAPCRAFT_PART_INSTALL/usr/lib/$SNAPCRAFT_ARCH_TRIPLET_BUILD_FOR/

but the following would not

override-build: |
    ./build.sh

build.sh

#!/bin/sh
mv lib.a $SNAPCRAFT_PART_INSTALL/usr/lib/$SNAPCRAFT_ARCH_TRIPLET_BUILD_FOR/

nor,

parts:
    source: .
    plugin: make

Makefile,

all:
    mv lib.a $SNAPCRAFT_PART_INSTALL/usr/lib/$(SNAPCRAFT_ARCH_TRIPLET_BUILD_FOR)/

[sergiusens]

always emit a warning for an empty build-for