acme/autocert: change a var to a const

A var isn't needed and a const is what the upstream (Go 1.10+) version
it's copying is.

Change-Id: I335270be3b3d09ac3c22cf6fb889a74ac39b8f1d
Reviewed-on: https://go-review.googlesource.com/116379
Reviewed-by: Filippo Valsorda <filippo@golang.org>

acme/autocert/autocert.go

@@ -276,7 +276,7 @@ func supportsECDSA(hello *tls.ClientHelloInfo) bool {
 		ecdsaOK := false
 	schemeLoop:
 		for _, scheme := range hello.SignatureSchemes {
-			var tlsECDSAWithSHA1 tls.SignatureScheme = 0x0203 // constant added in Go 1.10
+			const tlsECDSAWithSHA1 tls.SignatureScheme = 0x0203 // constant added in Go 1.10
 			switch scheme {
 			case tlsECDSAWithSHA1, tls.ECDSAWithP256AndSHA256,
 				tls.ECDSAWithP384AndSHA384, tls.ECDSAWithP521AndSHA512: