With Build agent tasks taking over things are different. What I do to see the various build environment variables is to make a simple batch file containing this:
SET C:\temp\EnvVars.txt
That'll produce a quick list of what is available.
Here's what I see with the TFS 2017 build agent:
agent.jobstatus=Succeeded
AGENT_BUILDDIRECTORY=C:\Agent\_work\2
AGENT_HOMEDIRECTORY=C:\Agent
AGENT_ID=2 AGENT_JOBNAME=Build
AGENT_JOBSTATUS=Succeeded
AGENT_MACHINENAME=BUILDMACHINE
AGENT_NAME=BUILDMACHINE
AGENT_OS=Windows_NT
AGENT_ROOTDIRECTORY=C:\Agent\_work
AGENT_SERVEROMDIRECTORY=C:\Agent\externals\vstsom
AGENT_TEMPDIRECTORY=C:\Agent\_work\_temp
AGENT_TOOLSDIRECTORY=C:\Agent\_work\_tool
AGENT_VERSION=2.122.1
AGENT_WORKFOLDER=C:\Agent\_work
BUILD_ARTIFACTSTAGINGDIRECTORY=C:\Agent\_work\2\a
BUILD_BINARIESDIRECTORY=C:\Agent\_work\2\b
BUILD_BUILDID=2036
BUILD_BUILDNUMBER=Database Build_20190708.2
BUILD_BUILDURI=vstfs:///Build/Build/2036
BUILD_CONTAINERID=2281
BUILD_DEFINITIONNAME=Database Build
BUILD_DEFINITIONVERSION=17
BUILD_QUEUEDBY=Smith, John
BUILD_QUEUEDBYID=8c588342-b87a-40cb-9b8c-a0ed10b57a3f
BUILD_REASON=Manual
BUILD_REPOSITORY_CLEAN=false
BUILD_REPOSITORY_GIT_SUBMODULECHECKOUT=False
BUILD_REPOSITORY_ID=$/
BUILD_REPOSITORY_LOCALPATH=C:\Agent\_work\2\s
BUILD_REPOSITORY_NAME=Collection
BUILD_REPOSITORY_PROVIDER=TfsVersionControl
BUILD_REPOSITORY_TFVC_WORKSPACE=ws_2_2
BUILD_REPOSITORY_URI=http://TFSSERVER:8080/tfs/Project/
BUILD_REQUESTEDFOR=Smith, John
BUILD_REQUESTEDFOREMAIL=John.Smith@Mailinator.com
BUILD_REQUESTEDFORID=7a588222-b66a-40ee-9b2a-a0ba10b12a3f
BUILD_SOURCEBRANCH=$/Collection/Project/Code
BUILD_SOURCEBRANCHNAME=Code
BUILD_SOURCESDIRECTORY=C:\Agent\_work\2\s
BUILD_SOURCEVERSION=9811
BUILD_SOURCEVERSIONAUTHOR=Smith, John
BUILD_SOURCEVERSIONMESSAGE=Added missing permission
BUILD_STAGINGDIRECTORY=C:\Agent\_work\2\a