update the INSTALL instructions for Windows to match changes of the Z…

…3 search path

reported by @GenericMonkey

INSTALL.md

@@ -19,12 +19,12 @@ normal Rust compiler.
 
 Change directory to `source`: `cd source`
 
-### On Windows: Make sure the Z3 executable is in your path
+### On Windows: Get Z3 and set the `VERUS_Z3_PATH` environment variable
 
 Download the [Z3 binaries](https://github.com/Z3Prover/z3/releases).
 Make sure you get Z3 4.12.2.
 The Z3 `bin` folder contain the executable `z3.exe`.
-Either add the Z3 `bin` folder to your path or copy the Z3 executable file to one of the folders in your path.
+Set the `VERUS_Z3_PATH` environment variable to the path of the Z3 executable file.
 
 ### On Unix/macOS/Windows: Get a local Z3