fix: disable omnitrail attestor on windows (in-toto#278)

Currently omnitrail expects a POSIX filesystem, which windows does not
supply. This causes windows builds to break when compiled with the
omnitrail attestor. This PR adds a build flag to skip the omnitrail
attestor when building for windows.

Signed-off-by: Mikhail Swift <mikhail@testifysec.com>

attestation/omnitrail/omnitrail.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !windows
+
 package omnitrail
 
 import (

imports.go

@@ -29,7 +29,6 @@ import (
 	_ "github.com/in-toto/go-witness/attestation/material"
 	_ "github.com/in-toto/go-witness/attestation/maven"
 	_ "github.com/in-toto/go-witness/attestation/oci"
-	_ "github.com/in-toto/go-witness/attestation/omnitrail"
 	_ "github.com/in-toto/go-witness/attestation/policyverify"
 	_ "github.com/in-toto/go-witness/attestation/product"
 	_ "github.com/in-toto/go-witness/attestation/sarif"

imports_nonwindows.go

@@ -0,0 +1,23 @@
+// Copyright 2024 Witness Contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !windows
+
+package witness
+
+// These imports will only be run when go-witness is built on a non-windows target OS
+import (
+	// all of the following imports are here so that each of the package's init functions run appropriately
+	_ "github.com/in-toto/go-witness/attestation/omnitrail"
+)