[lld][benchmark] LNT expects format_version as a string

[int3]

Summary

Not sure how (if) it was working before... but I needed to make
this change for things to work against my local LNT instance (from a
recent checkout of the LNT repo.)

lld/utils/benchmark.py

def runBench(bench):
     os.chdir('..')
     return ret
 
 def buildLntJson(benchmarks):
     start = datetime.datetime.utcnow().isoformat()
     tests = [runBench(b) for b in benchmarks]
     end = datetime.datetime.utcnow().isoformat()
     ret = {
-        'format_version' : 2,
+        'format_version' : '2',
         'machine' : { 'name' : args.machine },
         'run' : {
             'end_time' : start,
             'start_time' : end,
             'llvm_project_revision': args.revision
         },
         'tests' : tests
     }