fix: honour an empty NO_COLOR as unset

An empty NO_COLOR turned colour off, which no-color.org reads as unset.
FORCE_COLOR and CLICOLOR_FORCE now paint a pipe. --check with several
platforms asks which to check rather than "Install for:", and
--list-platforms says when nothing is detected here.

install.ps1

@@ -11,7 +11,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "ea5d767eeb7612312e6a8fbf5f67b578fdc746f2d34ccc46cf96b978be7424ce"
+    $defaultInstallSha256 = "739a28ac23d5b0d82d8324ecd7665b2501c71c64d60abba175575f7f16f24ddc"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }