Add possibility to re-use env variable with different values

[hbragge]

Consider the config below, where env variable E1 is set in two different configs to two different values. However, when accessing it from shell (note: not from bake) it will have same value in both places ("val2" in this case).

Project {
CustomConfig C1 {
Set E1, value: "val1", env: true
PreSteps {
CommandLine "echo E1: $E1"
}
DefaultToolchain GCC
}

CustomConfig C2 {
Set E1, value: "val2", env: true
PreSteps {
CommandLine "echo E1: $E1"
}
DefaultToolchain GCC
}

CustomConfig C3 {
Dependency config: C1
Dependency config: C2
DefaultToolchain GCC
}
}

Produces the output:
../envtest$ bake C3
Loading ../envtest/Project.meta
**** Building 1 of 3: envtest (C1) ****
echo E1: $E1
E1: val2
**** Building 2 of 3: envtest (C2) ****
echo E1: $E1
E1: val2
**** Building 3 of 3: envtest (C3) ****

[aschaal]

Fixed, will work with 2.28.0.