main/build: Use latest Go1.25 features if possible.

In order to avoid breaking backwards compatibility, newer versions of Go
toolchains automatically set GODEBUG flags to disable any changes that
are not strictly backwards compatible when compiling old code.  However,
it is often the case that older code will work properly with the new
features and security updates enabled and those updates are generally
desirable.

The existing code in the main module will all work properly with all
changes in Go1.25, so this adds a directive when building with Go1.25 or
newer to override and remove the default GODEBUG flags which disable
newer features and security updates that are not strictly backwards
compatible.  In other words, it ensure the new features and security
updates implemented in Go1.25 are enabled when building with Go 1.25 or
newer.

The specific GODEBUG flags removed are:

- `containermaxprocs=0`
- `decoratemappings=0`
- `tlssha1=1`
- `updatemaxprocs=0`
- `x509sha256skid=0`

Author: davecgh

debug.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2025 The Decred developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+// This file changes the default GODEBUG values when building with newer
+// releases of Go to enable as many of the new features and security updates
+// that are not strictly backwards compatible as possible.
+//
+// For reference, in order to avoid breaking backwards compatibility, newer
+// versions of Go toolchains automatically set GODEBUG flags to disable any
+// changes that are not strictly backwards compatible when compiling old code.
+// However, it is often the case that older code will work properly with the
+// new features and security updates enabled and those updates are generally
+// desirable.
+//
+// WARNING: Do not blindly update this with each new Go release.  It needs to
+// be analyzed with each new release before updating to ensure none of the
+// changes in the newer versions of Go that are disabled by default due to not
+// being striclty backwards compatibile will break the existing code.
+
+//go:build go1.25
+//go:debug default=go1.25
+
+package main