scons: Fix the default KVM_ISA setting.

The KVM_ISA setting was moved into a CONF dict, but the code which
ensured it had a default if there was no possible KVM hosting ISA was
still setting that variable in the base environment dict. This moves
the setting into the CONF dict instead.

Change-Id: I067c969dd761b2cdb098bcba6cd6a4b643d2d427
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/63752
Reviewed-by: Earl Ou <shunhsingou@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/cpu/kvm/SConsopts

@@ -30,7 +30,7 @@ from gem5_scons import warning
 import gem5_scons
 
 # ISA code can set this to indicate what ISA KVM can target.
-main.SetDefault(KVM_ISA='')
+main['CONF'].setdefault('KVM_ISA', '')
 
 with gem5_scons.Configure(main) as conf:
     # Check if we should enable KVM-based hardware virtualization. The