Python script that calls vcvarsall.bat fails from GitLab CI

I have a Python script that works from cmd.exe and powershell.exe but not from a GitLab runner job on Windows.

def build():
    for c in ["Debug", "Release"]:
        subprocess.check_call(
            f'vcvarsall.bat x64 && \
            cmake -G "NMake Makefiles" \
                -B{os.path.dirname(os.path.realpath(__file__))}/src/x64/{c} \
                -S{os.path.dirname(os.path.realpath(__file__))}/src \
                -DCMAKE_BUILD_TYPE={c} & \
            cmake --build {os.path.dirname(os.path.realpath(__file__))}/src/x64/{c}',
    )

if __name__ == "__main__":
    build()

If I run the script from cmd.exe or powershell.exe like so: python build.py, everything works as expected - vcvarsall.bat sets up the environment correctly so the subsequent cmake call has all of the required environment variables.

Running the script from a GitLab runner job on Windows using the .gitlab-ci.yml below fails.

build:
  stage: build
  script:
    - python build.py
  tags:
    - windows

In the GitLab UI job stdout, I can confirm vcvarsall.bat is being run and cmake after that:

In the CMakeError.log, the following error is reported:

The above link error makes me think that running vcvarsall.bat from the GitLab runner job is not completely configuring the environment - it is changing the environment, but not enough to make the subsequent cmake complete successfully. I can see many differences between the environment configured by vcvarsall.bat when executed from the GitLab runner job and cmd.exe.

For example, the left is the environment defined when running the script from the GitLab runner job, and the right from cmd.exe:

Why do these environmental discrepancies exist, and how can I fix them?