[chore] tidy up media manager, add calling func to errors, build-script improvements (#1835)

* media manager tidy-up: de-interface and remove unused PostDataFunc

Signed-off-by: kim <grufwub@gmail.com>

* remove last traces of media.Manager being an interface

Signed-off-by: kim <grufwub@gmail.com>

* update error to provide caller, allow tuneable via build tags

Signed-off-by: kim <grufwub@gmail.com>

* remove kim-specific build script changes

Signed-off-by: kim <grufwub@gmail.com>

* fix merge conflicts

Signed-off-by: kim <grufwub@gmail.com>

* update build-script to support externally setting build variables

Signed-off-by: kim <grufwub@gmail.com>

---------

Signed-off-by: kim <grufwub@gmail.com>

internal/gtserror/new_nocaller.go

@@ -0,0 +1,38 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+//go:build noerrcaller
+
+package gtserror
+
+import (
+	"errors"
+	"fmt"
+)
+
+// Caller returns whether created errors will prepend calling function name.
+const Caller = false
+
+// newAt is the same as New() but allows specifying calldepth.
+func newAt(_ int, msg string) error {
+	return errors.New(msg)
+}
+
+// newfAt is the same as Newf() but allows specifying calldepth.
+func newfAt(_ int, msgf string, args ...any) error {
+	return fmt.Errorf(msgf, args...)
+}