docker: Android - use Qt 5.15.2, drop QT_PREFIX env variable

Dockerfile.android

@@ -5,7 +5,7 @@ ARG ANDROID_NDK_REVISION=21d
 ARG ANDROID_NDK_HASH=bcf4023eb8cb6976a4c7cff0a8a8f145f162bf4d
 ARG ANDROID_SDK_REVISION=4333796
 ARG ANDROID_SDK_HASH=92ffee5a1d98d856634e8b71132e8a95d96c83a63fde1099be3d86df3106def9
-ARG QT_VERSION=5.15
+ARG QT_VERSION=5.15.2
 
 WORKDIR /opt/android
 ENV WORKDIR=/opt/android
@@ -19,7 +19,6 @@ ENV ANDROID_SDK_ROOT=${WORKDIR}/tools
 ENV JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
 ENV PATH=${JAVA_HOME}/bin:${PATH}
 ENV PREFIX=${WORKDIR}/prefix
-ENV QT_PREFIX=${WORKDIR}/Qt-${QT_VERSION}
 ENV TOOLCHAIN_DIR=${ANDROID_NDK_ROOT}/toolchains/llvm/prebuilt/linux-x86_64
 
 RUN apt-get update \
@@ -66,7 +65,7 @@ RUN git clone git://code.qt.io/qt/qt5.git -b ${QT_VERSION} --depth 1 \
     -no-sql-mysql \
     -opensource -confirm-license \
     -android-arch arm64-v8a \
-    -prefix ${QT_PREFIX} \
+    -prefix ${PREFIX} \
     -nomake tools -nomake tests -nomake examples \
     -skip qtwebengine \
     -skip qtserialport \
@@ -201,15 +200,15 @@ CMD set -ex \
     && cd build/Android/release \
     && cmake \
     -DCMAKE_TOOLCHAIN_FILE="${ANDROID_NDK_ROOT}/build/cmake/android.toolchain.cmake" \
-    -DCMAKE_PREFIX_PATH="${PREFIX};${QT_PREFIX}" \
-    -DCMAKE_FIND_ROOT_PATH="${PREFIX};${QT_PREFIX}" \
+    -DCMAKE_PREFIX_PATH="${PREFIX}" \
+    -DCMAKE_FIND_ROOT_PATH="${PREFIX}" \
     -DCMAKE_BUILD_TYPE=Release \
     -DARCH="armv8-a" \
     -DANDROID_NATIVE_API_LEVEL=${ANDROID_NATIVE_API_LEVEL} \
     -DANDROID_ABI="arm64-v8a" \
     -DANDROID_TOOLCHAIN=clang \
     -DBoost_USE_STATIC_RUNTIME=ON \
-    -DLRELEASE_PATH="${QT_PREFIX}/bin" \
+    -DLRELEASE_PATH="${PREFIX}/bin" \
     -DQT_ANDROID_APPLICATION_BINARY="monero-wallet-gui" \
     -DWITH_SCANNER=ON \
     ../../.. \