registry: Use plugin directory from the build system for relocateable Windows builds

Instead of guessing something based on preprocessor defines and magic.
21 jobs for backport-mr-232-into-1.16-plugins-relocateable in 66 minutes and 41 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Preparation
passed #490502
gst indent

00:00:47

passed #490501
manifest

00:00:37

 
  Build
passed #490508
build cerbero android universal

00:13:52

passed #490509
build cerbero cross win32

00:11:09

passed #490510
build cerbero cross win64

00:07:02

passed #490507
build cerbero fedora x86_64

00:07:28

passed #490512
gst-ios-12.2
build cerbero ios universal

00:34:04

passed #490511
gst-macos-10.14
build cerbero macos x86_64

00:07:02

passed #490503
build fedora x86_64

00:06:30

passed #490504
build nodebug fedora x86_64

00:05:41

passed #490505
build static fedora x86_64

00:13:05

passed #490506
build static nodebug fedora x86_64

00:12:09

 
  Test
passed #490513
check fedora

00:04:44

passed #490514
integration testsuites fedora

00:09:40

passed #490519
valgrind bad

00:19:12

passed #490516
valgrind base

00:22:45

passed #490515
valgrind core

00:09:38

passed #490520
valgrind ges

00:11:30

passed #490517
valgrind good

00:11:58

passed #490518
valgrind ugly

00:02:40

 
  Integrate
passed #490521
android universal examples

00:08:03