Update cppwinrt to build 2.0.210304.5 (flutter#24801)

This fixes an issue in CppWinRT related to std::terminate(); the tool
now uses abort() instead.

CppWinRT patch: microsoft/cppwinrt#868

This also fixes a couple bugs in the README.md:
* Uses tmp as the temp dir into which we unpack the nupkg archive rather
  than cppwinrt to avoid confusion with the containing directory, which is
  named cppwinrt.
* Fixes a typo around the directory from which the cipd command should
  be executed.

DEPS

@@ -564,7 +564,7 @@ deps = {
     'packages': [
       {
         'package': 'flutter/cppwinrt/win-amd64',
-        'version': 'build:2.0.210301.1'
+        'version': 'build:2.0.210304.5'
       }
     ],
     'condition': 'download_windows_deps',

tools/cipd/cppwinrt/.gitignore

@@ -1,5 +1,5 @@
 # Temp directory into which we unpack the .nupkg archive
-cppwinrt
+tmp
 
 # nupkg archives
 *.nupkg

tools/cipd/cppwinrt/README.md

@@ -29,10 +29,9 @@ To update the CIPD package, follow these steps:
    ```
    unzip microsoft.windows.cppwinrt.<version_number>.nupkg -d tmp
    ```
-4. `cd` into the `tmp` directory.
 4. Create the CIPD package:
    ```
-   cipd create --pkg-def ../cppwinrt-win-amd64.cipd.yaml`
+   cipd create --pkg-def cppwinrt-win-amd64.cipd.yaml
    ```
    The tool should output that the package was successfully uploaded and
    verified, including the package path and an identifier SHA.
@@ -42,7 +41,7 @@ To update the CIPD package, follow these steps:
    ```
 6. Verify the package was successfully created and tagged:
    ```
-   cipd describe flutter/cppwinrt/win-amd64 <new_version_sha>
+   cipd describe flutter/cppwinrt/win-amd64 -version <new_version_sha>
    ```
 7. Delete the archive and temp directory:
    ```

tools/cipd/cppwinrt/cppwinrt-win-amd64.cipd.yaml

@@ -2,7 +2,7 @@ package: flutter/cppwinrt/win-amd64
 description: C++/WinRT tool used for building WinRT headers for Windows UWP builds
 install_mode: copy
 # The temp directory into which the nupkg archive is extracted.
-root: cppwinrt
+root: tmp
 data:
   # Contents of the cppwinrt/ directory are at the root of the CIPD package.
   - dir: .