fix(windows-e2e): 'auto' sentinel for install-ref - powershell -File eats empty-string args

Run 31520267702 died in 3s: 'Missing an argument for parameter
InstallRef'. powershell.exe -File drops a "" argument from the command
line entirely, so the parameter binder saw -InstallRef followed by
-SetupExeUrl. Default both the workflow input and the script parameter
to 'auto' (= newest release tag) instead of empty.

.github/workflows/install-e2e-windows-run.yml

@@ -41,10 +41,10 @@ on:
         type: string
         default: desktop
       install-ref:
-        description: 'Ref to install as OLD (served as main while the installer runs). Empty = the newest release tag in the checkout.'
+        description: 'Ref to install as OLD (served as main while the installer runs). auto = the newest release tag in the checkout.'
         required: false
         type: string
-        default: ''
+        default: auto
       setup-exe-url:
         description: 'Bootstrap installer to install OLD with. Default: the latest published one — what a user downloads today.'
         required: false

tests/install/windows-desktop-gui-e2e.ps1

@@ -82,10 +82,12 @@ param(
     # i.e. what the user starts on. The published Hermes-Setup.exe carries
     # no commit pin (Pin { commit: None, branch: "main" }) -- it installs
     # whatever `main` points at, so staging OLD means serving it there.
-    # Empty = newest release tag in the checkout (the "user on the current
-    # release" starting point, same philosophy as the linux axis's tag
-    # matrix).
-    [string]$InstallRef = "",
+    # Empty or "auto" = newest release tag in the checkout (the "user on
+    # the current release" starting point, same philosophy as the linux
+    # axis's tag matrix). "auto" exists because `powershell -File` silently
+    # swallows an empty-string argument ('Missing an argument for
+    # parameter'), so the workflow cannot pass "".
+    [string]$InstallRef = "auto",
 
     # Repo checkout whose HEAD is the update target.
     [string]$RepoRoot = "",
@@ -301,7 +303,7 @@ function Invoke-PhaseStage {
     # OLD: explicit -InstallRef, or the newest release tag -- the version a
     # user who installed on release day is on.
     $oldRef = $InstallRef
-    if (-not $oldRef) {
+    if (-not $oldRef -or $oldRef -eq "auto") {
         $oldRef = (Invoke-Git @("-C", $RepoRoot, "tag", "--list", "v*", "--sort=-creatordate") -split "`r?`n" | Select-Object -First 1)
         if (-not $oldRef) { throw "no v* release tags in the checkout and no -InstallRef given -- cannot pick an OLD version" }
     }