twister: deal with variants set on command line

not force host variant if environment is setting something different in
the case of posix arch.

Fixes #83766

(cherry picked from commit 5769966)

Original-Signed-off-by: Anas Nashif <anas.nashif@intel.com>
GitOrigin-RevId: 5769966
Cr-Build-Id: 8726132470642320465
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8726132470642320465
Copybot-Job-Name: zephyr-main-copybot-downstream
Change-Id: I0adcdd707d690d3d8ddc6b881218fb5f6b140113
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/6167011
Commit-Queue: Keith Short <keithshort@chromium.org>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Keith Short <keithshort@chromium.org>
Tested-by: Keith Short <keithshort@chromium.org>

scripts/pylib/twister/twisterlib/testplan.py

@@ -898,9 +898,14 @@ def apply_filters(self, **kwargs):
             ):
                 if itoolchain:
                     toolchain = itoolchain
+                elif plat.arch in ['posix', 'unit']:
+                    # workaround until toolchain variant in zephyr is overhauled and improved.
+                    if self.env.toolchain in ['llvm']:
+                        toolchain = 'llvm'
+                    else:
+                        toolchain = 'host'
                 else:
-                    default_toolchain = "zephyr" if not self.env.toolchain else self.env.toolchain
-                    toolchain = default_toolchain if plat.arch not in ['posix', 'unit'] else "host"
+                    toolchain = "zephyr" if not self.env.toolchain else self.env.toolchain
 
                 instance = TestInstance(ts, plat, toolchain, self.env.outdir)
                 instance.run = instance.check_runnable(