[infra] Handle DART_EXPERIMENTAL_BUILD in tools/bots/dart-sdk.py.
[infra] Handle DART_EXPERIMENTAL_BUILD in tools/bots/dart-sdk.py.

This change uses the DART_EXPERIMENTAL_BUILD environment variable to
detect whether the current build happens to be experimental and
shouldn't have side effects.

Bug: b/215113476
Change-Id: Id5442d80d69309e320077f053d8af75b3aab670a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/228862
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: Jonas Termansen <092ec58085ae5e368bc39d18752740ff81a2d09c@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk