fix(@sandbox): greatly speed up (re)build times (#746)

since most of the compilation time is spent on creating the QRC bundle
and the resulting binary is quite huge, advice the `rcc` compiler to add
support for "big resources"
(cf https://doc.qt.io/qt-6/resources.html#embedding-large-files)

Also pass the env $LDFLAGS to qmake (to be able to link e.g. with mold,
which again greatly reduces the time to link the binary)

this leads to 6-10x faster time when recompiling
(Linux, gcc 12.1.1, mold 1.3.0)

BEFORE:
Clean build:
real	0m57,256s
user	1m25,869s
sys	0m40,963s

Incremental build, one file touched:
real	0m54,323s
user	0m54,012s
sys	0m2,934s

AFTER:
Clean build:
real	0m7,276s
user	0m21,001s
sys	0m37,256s

Incremental build, one file touched:
real	0m5,569s
user	0m4,850s
sys	0m0,840s

sandbox/sandbox.pro

@@ -1,11 +1,13 @@
 QT += quick svg
 
 CONFIG += c++11 warn_on qtquickcompiler
+CONFIG += resources_big
 
 QML_IMPORT_PATH= $$PWD/../src
 
 DEFINES += QT_DEPRECATED_WARNINGS
 
+QMAKE_LFLAGS += $$(LDFLAGS)
 
 # You can make your code fail to compile if it uses deprecated APIs.
 # In order to do so, uncomment the following line.