bug fix in build.py which prevents running some unit tests

[baijumeswani]

Description:
Argument to build.py --minimal_build is a string argument. But it is being used as a bool where there is a bug. So, some of the tests are not being run as they should be in the ci pipelines.

[baijumeswani]

Some more background:
It seems like this bug was introduced in #5776 here where the bool argument --minimal_build was changed to a string argument.
But while doing so, one place where this argument was used (introduced in #5056 here) was missed.
This led to the python tests from not being executed in the pipeline.

So far, tests would have passed but were not being executed in the pipeline.

Now, with changes from #5840, this line, would have caused one of the unit tests to fail (TestInferenceSession.testModelSerialization). But because these tests were not running in the pipeline, this failure was not caught.

From what I gather, this model serialization will throw an error for any execution provider that generate compiled nodes. And because of this, this test fails. So, I made the change to catch the exception thrown and compare it against the message "[ONNXRuntimeError] : 1 : FAIL : Unable to serialize model as it contains compiled nodes. Please disable any execution providers which generate compiled nodes." and pass the test if these two strings are equal. In short, this test should not be run when the exception thrown matches "[ONNXRuntimeError] : 1 : FAIL : Unable to serialize model as it contains compiled nodes. Please disable any execution providers which generate compiled nodes.".

[baijumeswani]

Thank you for the review. 😃