Use `eval` instead of re-executing `bash` in `tools/buildbot-build.sh`.

When evaluating a string to run it as a command in Bash script
`tools/buildbot-build.sh`, use the `eval` builtin instead of executing
another instance of the shell via `bash -c`.

Test: art/tools/buildbot-build.sh
Change-Id: Ia33c2fca7b94d41505107659724c18c5cb1b6ef9