meson/post-install.py: allow running without pkg-config (on Windows)

1. Prepend `gtk_bindir` to `PATH`. If GLib is built as a subproject, this will
allow running `glib-compile-schemas` and `gio-querymodules` without full path.

2. Don't fail when `pkg-config` is not available. Just try to run everything
without full paths.

Fixes GNOME/gtk#3723
11 jobs for fix-postinstall-no-pkgconfig in 29 minutes and 17 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  .Pre
passed style-check-diff #1211080

00:06:37

 
  Build
passed fedora-x86_64 #1211081

00:12:24

passed installed-tests #1211083

00:11:40

passed msys2-mingw64 #1211084
win32-ps

00:16:52

passed release-build #1211082

00:11:31

 
  Analysis
failed asan-build #1211086
asan allowed to fail

00:11:19

passed static-scan #1211085

00:10:25

 
  Docs
passed reference #1211087

00:09:33

 
  Flatpak
manual flatpak-manual:demo #1211088
flatpak allowed to fail manual
manual flatpak-manual:icon-browser #1211090
flatpak allowed to fail manual
manual flatpak-manual:widget-factory #1211089
flatpak allowed to fail manual
 
Name Stage Failure
failed
asan-build Analysis
 - css: 214 total, 212 pass, 2 fail
Processing gtk suite a11y:
- a11y: 22 total, 20 pass, 2 fail
Processing gtk suite tools:
- tools: 4 total, 3 pass, 1 fail
Processing gtk suite reftest:
- reftest: 134 total, 130 pass, 4 fail
Cleaning up file based variables
ERROR: Job failed: exit code 1