I’ve just spent a couple of hours figuring out how to get a Continua job to build an existing configuration when we have changed the MSBuild environment variables to point to Delphi 10.4 instead of Delphi 10.3. This was an exercise in frustration so I’m posting here in the hope that this will be useful for others. or myself when we move to a new version of Delphi in the future 
The error was that a file in a third party library could not be found “E:\CI_AWS\Ws\1213\Source\path_to_file\filename.pas(62): error F2613: Unit ‘ExceptionLog7’ not found.”
Logging into the build server using my AD credentials and running the build via the Delphi IDE was successful.
After much googling, I found reference to the EnvOptions.proj file which typically lives at C:\Users{ user account }\AppData\Roaming\Embarcadero\BDS\21. This file contains paths amongst other configuration details.
This file existed for my user account, but our Continua service runs under a service account. Looking at the location for that service account revealed it was missing for Delphi 10.4 (21) but present for Delphi 10.3 (20).
Copying the file across from my user account has fixed the problem.