fix: forward powershell style env casing (#6485)
### Description
Powershell populates `PATH` and `SYSTEMROOT` as `Path` and `SystemRoot`
causing our wildcarding to miss. These are valid populations as Windows
has case-insensitive environment variables. In both Go and Rust we were
explicit and forwarded `SYSTEMROOT`, but this failed on Powershell as
our wildcard matching is case sensitive. The Go standard library papered
over this failure since it [passes `SYSTEMROOT` for us if we don't set
it](https://pkg.go.dev/os/exec#Cmd):
> As a special case on Windows, SYSTEMROOT is always added if missing
and not explicitly set to the empty string.
Future work will be to investigate if we should make our environment
variable matching case insensitive on Windows to better match how
Windows thinks about environment variables.
### Testing Instructions
Make a repository with the following `build` script: `echo
%SYSTEMROOT%`. Verify that after this PR when running this script using
`turbo build --filter=foo --env-mode=strict` from Powershell now
displays `C:\Windows`:
Closes TURBO-1689
Co-authored-by: Chris Olszewski <Chris Olszewski>
