Text variables do not work in pcbnew

Description

Variables can not be used in text items in pcbnew. Current behavior: the variable name with "brace}" inserted after the open brace appears. Expected behavior: variable contents appear in text item.

Steps to reproduce

1. Start a new project
2. Open pcbnew
3. Add a new variable in File - Board setup... - Text variables, eg. TEST with value "Foo"
4. Select Text tool in pcbnew, click anywhere to add new text
5. Type "${TEST}" in dialog, hit OK
6. New text object with the string "${brace}TEST}" is created

KiCad Version

Application: Pcbnew

Version: (5.99.0-8225-g9a84d15965), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.68.0 GnuTLS/3.6.15 zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh/0.9.3/openssl/zlib nghttp2/1.41.0 librtmp/2.3

Platform: Linux 5.8.0-33-generic x86_64, 64 bit, Little endian, wxGTK, ubuntu, x11

Build Info:
	Date: Jan  7 2021 08:02:22
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCE: 6.9.1
	Curl: 7.68.0
	ngspice: 30
	Compiler: GCC 10.2.0 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON