Remove obsolete Bazel spawn strategies

After the latest Bazel upgrade, the --spawn_strategy=standalone
doesn't show the output of the subprocess created, making the
troubleshoot of the failures impossible.

Since release 0.27 Bazel auto detects the execution strategy, if no
strategy flag is provided. If none of the strategy flags was used,
Bazel will generate a default list of strategies (in this order):

   remote,worker,sandboxed,local

and, for every action it wants to execute, will pick up the first
strategy that can execute it.

See this blog entry for more details: [1].

[1] https://blog.bazel.build/2019/06/19/list-strategy.html

Change-Id: I4be8375cee88f3565bae5c53cd1a3484ce398aba

roles/bazelisk-build/tasks/main.yaml

@@ -5,8 +5,6 @@
     {{ bazelisk_executable }} version
     {{ bazelisk_executable }} \
       build \
-      --spawn_strategy=standalone \
-      --genrule_strategy=standalone \
       {{ bazelisk_targets|join(' ') }}
   args:
     executable: /bin/bash