oeqa/sdk/context: fix for gtk3 test failure during do_testsdk

oeqa/sdk/context: fix for gtk3 test failure during do_testsdk

The do_testsdk for lib32-core-image-sato aborts with below error:
configure: error: Package requirements (gtk+-3.0) were not met:
No package 'gtk+-3.0' found
Consider adjusting the PKG_CONFIG_PATH environment variable if you installed software in a non-standard prefix.

This causes due to an absolute path name in 'sdk_env', which is now stripped to have only the environment name.

Signed-off-by: Harish Sadineni <Harish.Sadineni@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>