hostinfo: use Distro field for distinguishing Windows Server builds

Some editions of Windows server share the same build number as their
client counterpart; we must use an additional field found in the OS
version information to distinguish between them.

Even though "Distro" has Linux connotations, it is the most appropriate
hostinfo field. What is Windows Server if not an alternate distribution
of Windows? This PR populates Distro with "Server" when applicable.

Fixes #11785

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

cmd/derper/depaware.txt

@@ -157,6 +157,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         tailscale.com/util/syspolicy                                 from tailscale.com/ipn
         tailscale.com/util/vizerror                                  from tailscale.com/tailcfg+
    W 💣 tailscale.com/util/winutil                                   from tailscale.com/hostinfo+
+   W 💣 tailscale.com/util/winutil/winenv                            from tailscale.com/hostinfo
         tailscale.com/version                                        from tailscale.com/derp+
         tailscale.com/version/distro                                 from tailscale.com/envknob+
         tailscale.com/wgengine/filter                                from tailscale.com/types/netmap

cmd/tailscale/depaware.txt

@@ -166,6 +166,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/util/vizerror                                  from tailscale.com/tailcfg+
      💣 tailscale.com/util/winutil                                   from tailscale.com/clientupdate+
    W 💣 tailscale.com/util/winutil/authenticode                      from tailscale.com/clientupdate
+   W 💣 tailscale.com/util/winutil/winenv                            from tailscale.com/hostinfo
         tailscale.com/version                                        from tailscale.com/client/web+
         tailscale.com/version/distro                                 from tailscale.com/client/web+
         tailscale.com/wgengine/capture                               from tailscale.com/cmd/tailscale/cli

cmd/tailscaled/depaware.txt

@@ -399,6 +399,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
      💣 tailscale.com/util/winutil                                   from tailscale.com/clientupdate+
    W 💣 tailscale.com/util/winutil/authenticode                      from tailscale.com/clientupdate+
    W    tailscale.com/util/winutil/policy                            from tailscale.com/ipn/ipnlocal
+   W 💣 tailscale.com/util/winutil/winenv                            from tailscale.com/hostinfo
         tailscale.com/util/zstdframe                                 from tailscale.com/control/controlclient+
         tailscale.com/version                                        from tailscale.com/client/web+
         tailscale.com/version/distro                                 from tailscale.com/client/web+

hostinfo/hostinfo_windows.go

@@ -13,18 +13,28 @@ import (
 	"golang.org/x/sys/windows/registry"
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/winutil"
+	"tailscale.com/util/winutil/winenv"
 )
 
 func init() {
+	distroName = lazyDistroName.Get
 	osVersion = lazyOSVersion.Get
 	packageType = lazyPackageType.Get
 }
 
 var (
+	lazyDistroName  = &lazyAtomicValue[string]{f: ptr.To(distroNameWindows)}
 	lazyOSVersion   = &lazyAtomicValue[string]{f: ptr.To(osVersionWindows)}
 	lazyPackageType = &lazyAtomicValue[string]{f: ptr.To(packageTypeWindows)}
 )
 
+func distroNameWindows() string {
+	if winenv.IsWindowsServer() {
+		return "Server"
+	}
+	return ""
+}
+
 func osVersionWindows() string {
 	major, minor, build := windows.RtlGetNtVersionNumbers()
 	s := fmt.Sprintf("%d.%d.%d", major, minor, build)