The Pre-SCM and Post-SCM environment scripts are not able to use any variables set up in the Pre-SCM and Post-SCM properties sections.

According to the Help in both sections:
Execute a script file aimed at setting an environment such a create folders, copying files, and so on.
Give the script file content.
You can use the above properties variables. <---NOT WORKING
However, adding or overriding environment variables in the script has no impact in the build job.

To reproduce (same result in both Pre-SCM section and Post-SCM section):
1) Create a new job and select configuration
2) Check "prepare an environment" for the job
3) In the Properties Content fill in
--------------------------------------------------------------
TESTVAR=whereAmI
--------------------------------------------------------------
4) In the Environment Script Content fill in:
---------------------------------------------------------------
echo ${TESTVAR}
echo $TESTVAR
echo %TESTVAR%
set
---------------------------------------------------------------

RESULT:
TESTVAR is not set/resolvable anywhere

LOG:
[EnvInject] - Injecting as environment variables the properties content
'TESTVAR=whereAmI
'

[EnvInject] - Executing the script:
echo ${TESTVAR}
echo $TESTVAR
echo %TESTVAR%
set
[jenkins] $ cmd /c call C:\WINDOWS\TEMP\hudson3574349175901208979.bat

D:\jenkins>echo ${TESTVAR}
${TESTVAR}

D:\jenkins>echo $TESTVAR
$TESTVAR

D:\jenkins>echo
ECHO is on.