net/interfaces: also allow link-local for AzureAppServices.

In May 2021, Azure App Services used 172.16.x.x addresses:
```
10: eth0@if11: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state UP
    link/ether 02:42:ac:10:01:03 brd ff:ff:ff:ff:ff:ff
    inet 172.16.1.3/24 brd 172.16.1.255 scope global eth0
       valid_lft forever preferred_lft forever
```

Now it uses link-local:
```
2: eth0@if6: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state UP
    link/ether 8a:30:1f:50:1d:23 brd ff:ff:ff:ff:ff:ff
    inet 169.254.129.3/24 brd 169.254.129.255 scope global eth0
       valid_lft forever preferred_lft forever
```

This is reasonable for them to choose to do, it just broke the handling in net/interfaces.

This PR proposes to:
1. Always allow link-local in LocalAddresses() if we have no better
   address available.
2. Continue to make isUsableV4() conditional on an environment we know
   requires it.

I don't love the idea of having to discover these environments one by
one, but I don't understand the consequences of making isUsableV4()
return true unconditionally. It makes isUsableV4() essentially always
return true and perform no function.

Fixes https://github.com/tailscale/tailscale/issues/7603

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/interfaces/interfaces.go

@@ -153,11 +153,9 @@ func LocalAddresses() (regular, loopback []netip.Addr, err error) {
 	if len(regular4) == 0 && len(regular6) == 0 {
 		// if we have no usable IP addresses then be willing to accept
 		// addresses we otherwise wouldn't, like:
-		//   + 169.254.x.x (AWS Lambda uses NAT with these)
+		//   + 169.254.x.x (AWS Lambda and Azure App Services use NAT with these)
 		//   + IPv6 ULA (Google Cloud Run uses these with address translation)
-		if hostinfo.GetEnvType() == hostinfo.AWSLambda {
-			regular4 = linklocal4
-		}
+		regular4 = linklocal4
 		regular6 = ula6
 	}
 	regular = append(regular4, regular6...)
@@ -645,7 +643,14 @@ func isUsableV4(ip netip.Addr) bool {
 		return false
 	}
 	if ip.IsLinkLocalUnicast() {
-		return hostinfo.GetEnvType() == hostinfo.AWSLambda
+		switch hostinfo.GetEnvType() {
+		case hostinfo.AWSLambda:
+			return true
+		case hostinfo.AzureAppService:
+			return true
+		default:
+			return false
+		}
 	}
 	return true
 }