[msbuild] Fix detecting already signed executables. (#3743)

[spouliot]

Note: backport from master / d15-8

The MSBuild tasks will codesign an executable if the executable's timestamp is
later than _CodeSignature/CodeResources's timestamp (or if
_CodeSignature/CodeResources doesn't exist).

Unfortunately, the codesign executable modifies both of those files, and the
executable last. This means that even just after running codesign, the
executable's timestamp might be later than _CodeSignature/CodeResources's
timestamp (due to HFS+'s one-second timestamp resolution, this might happen
all within the same second, which means that this is a random issue: the
problem only occurs if the executable was modified at least a second later
than _CodeSignature/CodeResources.)

So make sure to touch _CodeSignature/CodeResources after running codesign,
so that the next time a build occurs (with no modifications), we don't resign
needlessly.

Fixes this (random) test failure when running the MSBuild tests:

1) Test Failure : Xamarin.iOS.Tasks.TargetTests.RebuildExecutable_NoModifications
     #1: ../MySingleView/bin/iPhoneSimulator/Debug/MySingleView.app/MySingleView
  Expected: 2017-11-30 10:04:20.000
  But was:  2017-11-30 10:04:22.000

Fixes https://github.com/xamarin/maccore/issues/592.

[jstedfast]

Awesome catch, thanks!

[monojenkins]

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