Expose bamboo.basicBuildTimestamp in addition to the extended timestamp currently available

Per ISO8601, '2013-03-31T09:57:00.457+04:00' is the extended version of '20130332T095700.457+0400'; However, colons can be problematic in file and path operations, unless escaped, making it more difficult to handle such operations if build results or artifacts want to make use of the buildTimeStamp. Adding the basicBuildTimeStamp variable as built-in would make it simple to use this in any scenario.