build: set CMAKE_INSTALL_PREFIX explicitly

Recent changes to upstream CMakeFiles changed the default install
location.  Explicitly specify our desired install location.

Changed for Linux only; Windows was already doing this.

Change-Id: Id71945d426ac8f0a445dca3855bfa037870aaed9
diff --git a/update_external_sources.sh b/update_external_sources.sh
index 5440023..78f21f9 100755
--- a/update_external_sources.sh
+++ b/update_external_sources.sh
@@ -81,7 +81,7 @@
    cd "${BASEDIR}"/glslang
    mkdir -p build
    cd build
-   cmake -D CMAKE_BUILD_TYPE=Release ..
+   cmake -D CMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install ..
    make -j $CORE_COUNT
    make install
 }
@@ -91,7 +91,7 @@
    cd "${BASEDIR}"/spirv-tools
    mkdir -p build
    cd build
-   cmake -D CMAKE_BUILD_TYPE=Release ..
+   cmake -D CMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install ..
    make -j $CORE_COUNT
 }