dashboard: fix windows-arm-10 build failure

The build is failing with 0xc000007b, which is an architecture mismatch.
Instead, just set GOARM=7, which is aligned with the windows-arm-zx2c4
builder we are replacing.

For golang/go#47019

Change-Id: I1f2aa18f6cdeb0dca4b234a164b915f2148f3936
Reviewed-on: https://go-review.googlesource.com/c/build/+/345130
Trust: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -2267,7 +2267,6 @@ func init() {
 		},
 		env: []string{
 			"GOARCH=arm64",
-			"GOHOSTARCH=arm64",
 		},
 	})
 	addBuilder(BuildConfig{
@@ -2278,9 +2277,8 @@ func init() {
 			return atLeastGo1(goBranch, 17) && buildRepoByDefault(repo)
 		},
 		env: []string{
-			"GOARCH=arm",
+			"GOARCH=arm64",
 			"GOARM=7",
-			"GOHOSTARCH=arm",
 		},
 		KnownIssue: 47019,
 	})