Gerber job file Revision field doesn't apply text substitution

Description

When plotting gerbers with "Generate Gerber job file" option checked, the revision field from Page Settings is exported to the gbrjob file, but no text substitution is done. If the revision field has been defined using a text variable such as ${VERSION}, this expression gets put as is in the job file. It would IMHO make more sense if the gerber job Revision field would always contain exactly the same value that is shown visually in the drawing sheet information block Rev: ... area.

Steps to reproduce

1. Add text variable VERSION with value v1.0 in Board Setup -> Text Variables
2. Set Revision to ${VERSION} in Page Settings
3. Observe the drawing sheet information block correctly shows Rev: v1.0
4. Export Gerbers with "Generate Gerber job file" option checked
5. Observe the gbrjob file contains "Revision": "${VERSION}" under GeneralSpecs -> ProjectId

Example:

KiCad Version

Application: KiCad PCB Editor

Version: 6.0.0-d3dd2cf0fa~116~ubuntu21.10.1, release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.74.0 OpenSSL/1.1.1l zlib/1.2.11 brotli/1.0.9 libidn2/2.3.1 libpsl/0.21.0 (+libidn2/2.3.0) libssh/0.9.6/openssl/zlib nghttp2/1.43.0 librtmp/2.3

Platform: Linux 5.13.0-22-generic x86_64, 64 bit, Little endian, wxGTK, gnome-xmonad, x11

Build Info:
	Date: Dec 23 2021 14:23:46
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.74.0
	OCC: 7.5.2
	Curl: 7.74.0
	ngspice: 35
	Compiler: GCC 11.2.0 with C++ ABI 1016

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON