fix: correct Compute Engine environment for DirectPath

[mohanli-ml]

As requested by #1323, we need further correct the Compute Engine environment check logic.

[Do not submit] We need to verify this PR in the produced bug before merge it.

[codecov]

Codecov Report

Merging #1325 (0fc0275) into master (27d92c6) will decrease coverage by 0.02%.
The diff coverage is 40.00%.

@@             Coverage Diff              @@
##             master    #1325      +/-   ##
============================================
- Coverage     79.65%   79.62%   -0.03%     
+ Complexity     1248     1247       -1     
============================================
  Files           209      209              
  Lines          5461     5458       -3     
  Branches        464      464              
============================================
- Hits           4350     4346       -4     
  Misses          928      928              
- Partials        183      184       +1     

Impacted Files	Coverage Δ	Complexity Δ
...api/gax/grpc/InstantiatingGrpcChannelProvider.java	77.14% <40.00%> (-0.33%)	35.00 <0.00> (ø)
.../java/com/google/api/gax/batching/BatcherImpl.java	94.38% <0.00%> (-0.57%)	21.00% <0.00%> (-1.00%)

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[elharo]

This must have a test that fails without the patch. This is not optional.