CMake: Don't use mkspec set by sourcing b2qt environment script

qmake sees the QMAKESPEC variable and uses it when building tests,
instead of using the target mkspec written in the target_qt.conf file
by the build system. Unset the environment variable to force usage
of the mkspec specified when confugring Qt.

Task-number: QTBUG-86053
Change-Id: I24ceddd1436393b496665e439dd1441ecbd36df7
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

coin/instructions/prepare_building_env.yaml

@@ -233,8 +233,9 @@ instructions:
 
   - type: Group
     instructions:
+    # Need to unset QMAKESPEC, so that the pre-installed boot2qt mkspec is not picked up.
     - type: WriteFile
-      fileContents: "#!/bin/bash\nunset LD_LIBRARY_PATH\n. {{.Env.QT_YOCTO_ENVSETUP}}\nexport PATH={{.Env.QT_CMAKE_DIR}}:$PATH;\n$*"
+      fileContents: "#!/bin/bash\nunset LD_LIBRARY_PATH\n. {{.Env.QT_YOCTO_ENVSETUP}}\nexport PATH={{.Env.QT_CMAKE_DIR}}:$PATH;\nunset QMAKESPEC\n$*"
       filename: "{{.Env.HOME}}/prefix.sh"
       fileMode: 493
       maxTimeInSeconds: 20