Interpolate environmental variable into a string

In a <VirtualHost> block I have this:

# Extract the first subdir and assign it to "subdir"
SetEnvIfNoCase Request_URI "^/?([^/]*)(?:/|$)" subdir=$1

# Add the subdir to the logging path
CustomLog "|bin/rotatelogs.exe -l c:/logs/%Y.%m_apache_%{ENV:subdir}_access.log 86400" combined

But it's not creating the desired:

c:\logs\apache_app1_access.log
c:\logs\apache_app2_access.log
c:\logs\apache_app3_access.log

I see this error in the error log:

AH00106: piped log program 'bin/rotatelogs.exe -l c:/logs/%Y.%m_apache_%{ENV:subdir}_access.log 86400' failed unexpectedly
Invalid filename given

Removing this section allows it to work:

_%{ENV:subdir}

Changing _%{ENV:subdir} into _${ENV:subdir} fails as well.