Use System Environment variables in .project file

Question

I have a demo C project where I have a linked resource file relative to a variable defined in the variableList section , FIRMWARE_DIR_FROM_ENV The value of FIRMWARE_DIR_FROM_ENV I want to be updated dynamically from the environment. I have tried to use file:/$%7BFIRMWARE_DIR_FROM_ENV%7D or file:/%FIRMWARE_DIR_FROM_ENV% but with no success.

If the environment variable FIRMWARE_DIR_FROM_ENV=C:/DEMO_LIBS then my expectation is to have on the end the file src/main.c linked to C:/DEMO_LIBS/src/main.c

Here is the .project file

<?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
<name>TestDemoProject</name>
<comment></comment>
<projects>
</projects>
<buildSpec>
    <buildCommand>
        <name>org.eclipse.cdt.managedbuilder.core.genmakebuilder</name>
        <triggers>clean,full,incremental,</triggers>
        <arguments>
        </arguments>
    </buildCommand>
    <buildCommand>
        <name>org.eclipse.cdt.managedbuilder.core.ScannerConfigBuilder</name>
        <triggers>full,incremental,</triggers>
        <arguments>
        </arguments>
    </buildCommand>
</buildSpec>
<natures>
    <nature>org.eclipse.cdt.core.cnature</nature>
    <nature>org.eclipse.cdt.managedbuilder.core.managedBuildNature</nature>
    <nature>org.eclipse.cdt.managedbuilder.core.ScannerConfigNature</nature>
</natures>
<linkedResources>
    <link>
        <name>src/main.c</name>
        <type>1</type>
        <locationURI>FIRMWARE_DIR_FROM_ENV/src/main.c</locationURI>
    </link>
</linkedResources>
<variableList>
    <variable>
        <name>FIRMWARE_DIR_FROM_ENV</name>
        <value>file:/%FIRMWARE_DIR_FROM_ENV%</value>
    </variable>
</variableList>

Thank you for your advice!