### What this change is doing?
This change aims to enable us to set the `VENDORED_PATH` as an environment variable and failback to the default value if the environment variable is not set. Thus, this change should not break current behavior.
### Why we need this?
This will enable using the script to build libgit2 form source code downloaded manually.
Example:
```sh
LIBGIT2_VER="1.0.1"
DOWNLOAD_URL="https://codeload.github.com/libgit2/libgit2/tar.gz/v${LIBGIT2_VER}""
LIBGIT2_PATH="${HOME}/libgit2-${LIBGIT2_VER}"
wget -O "${LIBGIT2_PATH}.tar.gz" "$DOWNLOAD_URL"
tar -xzvf "${LIBGIT2_PATH}.tar.gz"
VENDORED_PATH=$LIBGIT2_PATH sh ./script/build-libgit2.sh --static
```
(cherry picked from commit f3a746d7b6)
Suhaib Mujahid