riscv: Toolchain check triggers even with BUILD_IN_DOCKER

[chrysn]

Description

When building on a system that has no relevant toolchains installed, a test is run outside the container even on docker builds, printing:

No RISC-V toolchain detected. Make sure a RISC-V toolchain is installed.

Steps to reproduce the issue

Install a bare system (without any riscv*-gcc installed). Run:

$ make -C examples/hello-world  BUILD_IN_DOCKER=1 BOARD=hifive1b
make: Entering directory '/tmp/RIOT/examples/hello-world'
/tmp/RIOT/makefiles/arch/riscv.inc.mk:37: *** No RISC-V toolchain detected. Make sure a RISC-V toolchain is installed..  Stop.
make: Leaving directory '/tmp/RIOT/examples/hello-world'

Expected results

The test should be performed inside the actual build environment, not outside.

[edit: Assigning to whomever git blame reported, as I'm currently tracking down different urgent issues to get CI running again]