pin libmongocrypt to 1.5.0-alpha2 (#1018)

Author: kevinAlbs

.evergreen/compile-unix.sh

@@ -227,7 +227,7 @@ pkg-config --modversion libssl || true
 
 if [ "$COMPILE_LIBMONGOCRYPT" = "ON" ]; then
    # Build libmongocrypt, using the previously fetched installed source.
-   git clone https://github.com/mongodb/libmongocrypt
+   git clone https://github.com/mongodb/libmongocrypt --branch 1.5.0-alpha2
    mkdir libmongocrypt/cmake-build
    cd libmongocrypt/cmake-build
    $CMAKE -DENABLE_SHARED_BSON=ON -DCMAKE_BUILD_TYPE="Debug" -DCMAKE_INSTALL_PREFIX="$INSTALL_DIR" -DCMAKE_PREFIX_PATH="$CMAKE_PREFIX_PATH" ../

.evergreen/compile-windows.sh

@@ -115,7 +115,7 @@ fi
 
 if [ "$COMPILE_LIBMONGOCRYPT" = "ON" ]; then
    # Build libmongocrypt, using the previously fetched installed source.
-   git clone https://github.com/mongodb/libmongocrypt
+   git clone https://github.com/mongodb/libmongocrypt --branch 1.5.0-alpha2
    mkdir libmongocrypt/cmake-build
    cd libmongocrypt/cmake-build
    "$CMAKE" -G "$CC" "-DCMAKE_PREFIX_PATH=${INSTALL_DIR}/lib/cmake" -DENABLE_SHARED_BSON=ON -DCMAKE_INSTALL_PREFIX="$INSTALL_DIR" ../