Fix stack-use-after-scope reading QT_QPA_PLATFORM env var

[Smjert]

Description

Building with Asan on Linux and starting qrenderdoc causes an immediate crash.

Setting QT_QPA_PLATFORM with putenv from a local variable causes the environment to later still refer to it when it's out of scope. Use setenv instead, as it copies the values provided.

Asan output:

=================================================================
==186532==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7fad55b1acf0 at pc 0x7fad62676cb1 bp 0x7ffed4d0bda0 sp 0x7ffed4d0b548
READ of size 4 at 0x7fad55b1acf0 thread T0
    #0 0x7fad62676cb0 in strlen /usr/src/debug/gcc/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:391
    #1 0x7fad5b13fb10 in QByteArray::QByteArray(char const*, int) (/usr/lib/libQt5Core.so.5+0x13fb10) (BuildId: ea54d08e6101ccb20735d0238686c8f8aab9e1f8)
    #2 0x7fad5b0e37dc in qgetenv(char const*) (/usr/lib/libQt5Core.so.5+0xe37dc) (BuildId: ea54d08e6101ccb20735d0238686c8f8aab9e1f8)
    #3 0x7fad5b73208d in QGuiApplicationPrivate::createPlatformIntegration() (/usr/lib/libQt5Gui.so.5+0x13208d) (BuildId: 4303ae81f380b376ed634b87a7d226266b7f1eba)
    #4 0x7fad5b733b10 in QGuiApplicationPrivate::createEventDispatcher() (/usr/lib/libQt5Gui.so.5+0x133b10) (BuildId: 4303ae81f380b376ed634b87a7d226266b7f1eba)
    #5 0x7fad5b2b682c in QCoreApplicationPrivate::init() (/usr/lib/libQt5Core.so.5+0x2b682c) (BuildId: ea54d08e6101ccb20735d0238686c8f8aab9e1f8)
    #6 0x7fad5b733bc6 in QGuiApplicationPrivate::init() (/usr/lib/libQt5Gui.so.5+0x133bc6) (BuildId: 4303ae81f380b376ed634b87a7d226266b7f1eba)
    #7 0x7fad5bf53735 in QApplicationPrivate::init() (/usr/lib/libQt5Widgets.so.5+0x153735) (BuildId: 7b13dd81cdd603e649fac765c9b95098c6cc82b8)
    #8 0x5e534c7feab5 in main ../../qrenderdoc/Code/qrenderdoc.cpp:317
    #9 0x7fad5aa35487  (/usr/lib/libc.so.6+0x27487) (BuildId: 695cfc6aac7d0f77bb7caba0ef01b2e868762b02)
    #10 0x7fad5aa3554b in __libc_start_main (/usr/lib/libc.so.6+0x2754b) (BuildId: 695cfc6aac7d0f77bb7caba0ef01b2e868762b02)
    #11 0x5e534bc62144 in _start (/home/smjert/Development/renderdoc/build/bin/qrenderdoc+0xed1144) (BuildId: 000f3714b99c3395df4f6bd63614aa068280c136)

Address 0x7fad55b1acf0 is located in stack of thread T0 at offset 11504 in frame
    #0 0x5e534c7fd0f3 in main ../../qrenderdoc/Code/qrenderdoc.cpp:184

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    [11488, 11509) 'env_set' (line 206) <== Memory access at offset 11504 is inside this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-use-after-scope /usr/src/debug/gcc/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:391 in strlen
Shadow bytes around the buggy address:
  0x7fad55b1aa00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1aa80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1ab00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1ab80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1ac00: 00 00 00 00 00 00 00 00 00 00 00 f2 f2 f2 f2 f2
=>0x7fad55b1ac80: f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f8 f8[f8]f3
  0x7fad55b1ad00: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1ad80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1ae00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1ae80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7fad55b1af00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==186532==ABORTING