Use Ninja in swiftshader build script.

* We already require it for the main cmake side, so seems harmless to require it here too. Speeds up my builds by many minutes.
* Fix a couple of small bugs in the doc.

Closes https://github.com/google/iree/pull/1577

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/1577 from google:swiftbuild1 94328e5b44414940377f6153457e825ff6595955
PiperOrigin-RevId: 307649960
diff --git a/docs/GetStarted/getting_started_linux_vulkan.md b/docs/GetStarted/getting_started_linux_vulkan.md
index 5416502..557645d 100644
--- a/docs/GetStarted/getting_started_linux_vulkan.md
+++ b/docs/GetStarted/getting_started_linux_vulkan.md
@@ -48,7 +48,7 @@
 
 ```shell
 # -- CMake --
-$ set VK_LOADER_DEBUG=all
+$ export VK_LOADER_DEBUG=all
 $ cmake --build build/ --target iree_hal_vulkan_dynamic_symbols_test
 $ ./build/iree/hal/vulkan/iree_hal_vulkan_dynamic_symbols_test
 
@@ -64,7 +64,7 @@
 
 ```shell
 # -- CMake --
-$ set VK_LOADER_DEBUG=all
+$ export VK_LOADER_DEBUG=all
 $ cmake --build build/ --target iree_hal_cts_device_creation_test
 $ ./build/iree/hal/cts/iree_hal_cts_device_creation_test
 
@@ -112,7 +112,7 @@
 loader uses the ICD:
 
 ```shell
-$ VK_ICD_FILENAMES=$PWD/build-swiftshader/Linux/vk_swiftshader_icd.json
+$ export VK_ICD_FILENAMES=$PWD/build-swiftshader/Linux/vk_swiftshader_icd.json
 ```
 
 ### Setting up Vulkan-ExtensionLayer
@@ -135,10 +135,10 @@
 
 ```shell
 # -- CMake --
-$ VK_LAYER_PATH=$PWD/build/third_party/vulkan_extensionlayer/layers/:$VK_LAYER_PATH
+$ export VK_LAYER_PATH=$PWD/build/third_party/vulkan_extensionlayer/layers/:$VK_LAYER_PATH
 
 # -- Bazel --
-$ VK_LAYER_PATH=$PWD/bazel-bin/external/vulkan_extensionlayer/:$VK_LAYER_PATH
+$ export VK_LAYER_PATH=$PWD/bazel-bin/external/vulkan_extensionlayer/:$VK_LAYER_PATH
 ```
 
 ### Support in Bazel Tests