Fix Qt's rcc determinism for depends/gitian

Backport of https://bugreports.qt.io/browse/QTBUG-62511 to resolve
locale determinism during the build process.

contrib/gitian-descriptors/gitian-linux.yml

@@ -43,6 +43,7 @@ script: |
   HOST_LDFLAGS=-static-libstdc++
 
   export QT_RCC_TEST=1
+  export QT_RCC_SOURCE_DATE_OVERRIDE=1
   export GZIP="-9n"
   export TAR_OPTIONS="--mtime="$REFERENCE_DATE\\\ $REFERENCE_TIME""
   export TZ="UTC"

contrib/gitian-descriptors/gitian-osx.yml

@@ -40,6 +40,7 @@ script: |
   FAKETIME_PROGS="ar ranlib date dmg genisoimage"
 
   export QT_RCC_TEST=1
+  export QT_RCC_SOURCE_DATE_OVERRIDE=1
   export GZIP="-9n"
   export TAR_OPTIONS="--mtime="$REFERENCE_DATE\\\ $REFERENCE_TIME""
   export TZ="UTC"

contrib/gitian-descriptors/gitian-win.yml

@@ -36,6 +36,7 @@ script: |
   HOST_CXXFLAGS="-O2 -g"
 
   export QT_RCC_TEST=1
+  export QT_RCC_SOURCE_DATE_OVERRIDE=1
   export GZIP="-9n"
   export TAR_OPTIONS="--mtime="$REFERENCE_DATE\\\ $REFERENCE_TIME""
   export TZ="UTC"