ci: Set ZIG_GLOBAL_CACHE_DIR and ZIG_LOCAL_CACHE_DIR earlier.

Otherwise the disk just keeps getting filled up.

Also remove some pointless cleanup commands since the actions/checkout workflow
step already cleans the repository by default.

ci/x86_64-windows-debug.ps1

@@ -30,17 +30,16 @@ if ((git rev-parse --is-shallow-repository) -eq "true") {
     git fetch --unshallow # `git describe` won't work on a shallow repo
 }
 
-Write-Output "Building from source..."
-Remove-Item -Path 'build-debug' -Recurse -Force -ErrorAction Ignore
-New-Item -Path 'build-debug' -ItemType Directory
-Set-Location -Path 'build-debug'
-
 # Override the cache directories because they won't actually help other CI runs
 # which will be testing alternate versions of zig, and ultimately would just
 # fill up space on the hard drive for no reason.
 $Env:ZIG_GLOBAL_CACHE_DIR="$(Get-Location)\zig-global-cache"
 $Env:ZIG_LOCAL_CACHE_DIR="$(Get-Location)\zig-local-cache"
 
+Write-Output "Building from source..."
+New-Item -Path 'build-debug' -ItemType Directory
+Set-Location -Path 'build-debug'
+
 # CMake gives a syntax error when file paths with backward slashes are used.
 # Here, we use forward slashes only to work around this.
 & cmake .. `