fix(linux): enable Solar2DBuilder Android builds on Linux

[shtse8]

Fixes #890

What

Enables Solar2DBuilder to build Android APKs on Linux. The C++ code already had Rtt_LINUX_ENV defined and compiles fine — it just had Mac/Windows branches without a Linux equivalent in a few places.

Changes (5 files)

File	Fix
CMakeLists.txt	Simplify root cmake — was duplicating platform logic that belongs in platform/linux/CMakeList.txt
platform/linux/CMakeList.txt	Add Rtt_AndroidSupportTools.c to Solar2DBuilder sources; add CORONABUILDER_ANDROID compile definition
Rtt_AppPackagerAndroidFactory.cpp	Add Rtt_LINUX_ENV branch for AndroidValidation.lua path (was Mac/Win only)
Rtt_AppPackagerFactory.cpp	Implement GetResourceDirectory() for Linux via /proc/self/exe readlink
tools/CoronaBuilder/main.cpp	Use $TMPDIR instead of hardcoded /TemporaryFiles (root-owned on Linux CI)

Tested

Successfully builds Android APKs on Ubuntu 22.04 self-hosted runners. We've been using this in production CI for our Solar2D games.

Community build repo with pre-built binaries + auto-update workflow: https://github.com/Cubeage/solar2d-linux

[CLAassistant]

All committers have signed the CLA.

[chkuendig]

Heads up: #936 covers this ground, opened as a supersession rather than a competing patch.

The overlap is the Linux GetResourceDirectory(), the AndroidValidation.lua path branch, and the $TMPDIR fix. The $TMPDIR one is your find — a root-owned /TemporaryFiles is not a failure I would have guessed at.

Three deliberate differences:

• Linux: enable HTML5 and Android builds, fix display.save() colours #936 also installs AndroidValidation.lua. This PR adds the Linux branch that points at it, but nothing copies it into Resources/, so the path resolves to a file that is not there. Linux: enable HTML5 and Android builds, fix display.save() colours #936 adds the file(COPY ...) to platform/linux/CMakeResources.txt.
• GetResourceDirectory() reuses the existing GetStartupPath() from Rtt_LinuxUtils.h rather than open-coding the /proc/self/exe readlink.
• Linux: enable HTML5 and Android builds, fix display.save() colours #936 leaves the root CMakeLists.txt alone. The rewrite here drops CORONA_ROOT, BUILD_NUMBER and YEAR, and platform/linux/CMakeList.txt uses ${CORONA_ROOT} — including for the resource copies — so it would be unset.

#936 additionally gates the HTML5 builder and fixes display.save() colours on Linux, which are outside what this PR set out to do.