fix: use nested Join-Path for PowerShell 5.1 compatibility

[edenfunf]

Description

Fixes #592

apm runtime setup fails immediately on Windows with the default PowerShell 5.1 because the setup scripts call Join-Path with three or more positional path segments -- a syntax that requires PowerShell 6.0+.

Join-Path : A positional parameter cannot be found that accepts argument 'runtimes'.
At setup-common.ps1:27
+     $runtimeDir = Join-Path $env:USERPROFILE ".apm" "runtimes"

Windows ships with PowerShell 5.1 as the default shell. Users who have not separately installed PowerShell 7+ cannot set up any runtime.

The fix replaces every multi-segment Join-Path call with nested two-argument calls, which are compatible with both PowerShell 5.1 and 7+:

# Before (PS 6+ only)
$runtimeDir = Join-Path $env:USERPROFILE ".apm" "runtimes"

# After (PS 5.1+)
$runtimeDir = Join-Path (Join-Path $env:USERPROFILE ".apm") "runtimes"

Type of change

• Bug fix

Testing

• Tested locally
• All existing tests pass

Confirmed on Windows 11 Pro with PowerShell 5.1.26100.x:

• Reproduced the error using the original code (Join-Path A B C raises PositionalParameterNotFound on PS 5.1)
• Verified the fix produces the correct path with nested calls
• Full unit suite: 3614 tests passed