buildman/toolchain.py: do not set CROSS_COMPILE for sandbox

buildman/toolchain.py: do not set CROSS_COMPILE for sandbox

When building for sandbox, self.cross is empty.

In MakeEnvironment(), CROSS_COMPILE is defined to be self.cross (with
or without a full path), optionally prefixed by the toolchain wrapper
defined in ~/.buildman. This is fine when self.cross is not empty, but
it doesn't make sense when it is:
- Either there is no wrapper and we end up with an empty CROSS_COMPILE
which is the same as not defining it (the host compiler will be used),
- Or there is a wrapper and CROSS_COMPILE will contain only the wrapper
which obviously is not a valid compiler, hence an error.

Test case:

 $ sudo apt install ccache
 $ grep -q toolchain-wrapper ~/.buildman || \
     printf "[toolchain-wrapper]\nwrapper = ccache\n" >>~/.buildman
 $ make mrproper
 $ ./tools/buildman/buildman sandbox_noinst
 $ ./tools/buildman/buildman sandbox_noinst
 Building current source for 1 boards (1 thread, 24 jobs per thread)
    sandbox:  +   sandbox_noinst
 +arch/sandbox/lib/reloc_sandbox_efi.c:10:15: error: operator '==' has no left operand
 +   10 | #if HOST_ARCH == HOST_ARCH_X86_64
 +      |               ^~
[...]

The GetEnvArgs function is modified too, since the VAR_CROSS_COMPILE
case has the same issue.

In tools/buildman/test.py, testGetEnvArgs is extended and
testMakeEnvironment is added. They check the 'arm' and 'sandbox'
toolchains, with and without a wrapper.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>