Limit github checkout time in bazel-testing org (#2604)

Adding variables to the environment to limit git minimum speed:
GIT_HTTP_LOW_SPEED_LIMIT: "102400" (Sets a minimum speed threshold of
100 KB/s)
GIT_HTTP_LOW_SPEED_TIME: "180" (Sets a time limit of 180 seconds / 3
minutes)

If the Git download speed drops below 100 KB/s for a continuous 3
minutes during the checkout phase, Git will automatically abort the
operation and fail the build. This prevents the job from hanging
indefinitely on a stalled or extremely slow network connection in the
"Preparing working directory" step.