[Nova] Windows Adopt ffmpeg build to be executed from github actions (#3193)

Summary:
Adopt ffmpeg build to be executed from github actions for windows

Tested by manually invoking this script:
```
c:\actions-runner\_work\test-infra\test-infra\pytorch\audio
Chocolatey v1.2.1
Installing the following packages:
msys2
By installing, you accept licenses for the packages.
msys2 v20230318.0.0 already installed.
 Use --force to reinstall, specify a version to install, or try upgrade.

Chocolatey installed 0/1 packages.
 See the log for details (C:\ProgramData\chocolatey\logs\chocolatey.log).

Warnings:
 - msys2 - msys2 v20230318.0.0 already installed.
 Use --force to reinstall, specify a version to install, or try upgrade.

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warning: base-devel-...

packaging/ffmpeg/build.bat

 @echo off
 
 if exist "third_party\ffmpeg\" goto end
+set PROJ_FOLDER=%cd%
 
 choco install -y --no-progress msys2 --package-parameters "/NoUpdate"
 C:\tools\msys64\usr\bin\env MSYSTEM=MINGW64 /bin/bash -l -c "pacman -S --noconfirm --needed base-devel mingw-w64-x86_64-toolchain diffutils"
-C:\tools\msys64\usr\bin\env MSYSTEM=MINGW64 /bin/bash -l -c "cd /c/Users/circleci/project && export FFMPEG_ROOT=${PWD}/third_party/ffmpeg && packaging/vc_env_helper.bat bash ./packaging/ffmpeg/build.sh"
+C:\tools\msys64\usr\bin\env MSYSTEM=MINGW64 /bin/bash -l -c "cd ${PROJ_FOLDER} && export FFMPEG_ROOT=${PWD}/third_party/ffmpeg && packaging/vc_env_helper.bat bash ./packaging/ffmpeg/build.sh"
 
 :end