tool_homedir: Change GetEnv() to use libcurl's curl_getenv()

- Deduplicate GetEnv() code.

- On Windows change ultimate call to use Windows API
  GetEnvironmentVariable() instead of C runtime getenv().

Prior to this change both libcurl and the tool had their own GetEnv
which over time diverged. Now the tool's GetEnv is a wrapper around
curl_getenv (libcurl API function which is itself a wrapper around
libcurl's GetEnv).

Furthermore this change fixes a bug in that Windows API
GetEnvironmentVariable() is called instead of C runtime getenv() to get
the environment variable since some changes aren't always visible to the
latter.

Reported-by: Christoph M. Becker

Fixes https://github.com/curl/curl/issues/4774
Closes https://github.com/curl/curl/pull/4863

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