This can be indeed related to the environment change, specifically August 1st images update. Please try to set image to the Previous Visual Studio 2017 and see if this helps.
But please come back to us in any case. Even if switching to "previous" helps, it can be treated only as a temporary workaround, valid only till next update. So we will be needed to root cause it together.
I took a look at one specific failure you sent, this one. It fails because The system cannot find the file '..\GitExtensionsShellEx\Release\GitExtensionsShellEx32.dll'. However before it builds only 64-bit version of that file: C:\projects\gitextensions\GitExtensionsShellEx\Release-x64\GitExtensionsShellEx64.dll. On the good build it builds both 32- and 64-bit version of GitExtensionsShellEx. Also it seems that "good" build rise a warning warning C4996: 'GetVersion': was declared deprecated while "bad" not. Does it rings any bells?