Update INSTALL.md to Reflect Changes for Native Windows libsecp256k1 Dependency Setup

[boskowski]

This PR updates the installation instructions in INSTALL.md to replace the outdated steps for configuring libsodium.dll with steps to configure libsecp256k1.dll.

Summary of Changes:

• The original instructions described downloading libsodium.dll from nuget.org and placing it in C:\Windows\System. However, the libsodium.dll library is no longer compatible with bitcointx.
• The updated instructions guide users to:
  1. Install the libsecp256k1-0 Python package.
  2. Locate the compiled libsecp256k1-0.dll within the Python environment.
  3. Copy and rename it to secp256k1.dll in the Scripts folder of the Python environment.

Why This Change is Necessary:

The previous method for handling dependencies is no longer compatible with the current requirements of bitcointx, which is a critical component of Joinmarket. This change ensures that users can correctly configure the required library and continue using the software without encountering compatibility issues.

Additional Notes:

• The new instructions provide a simpler and more Python-environment-focused approach, avoiding the need for admin privileges to place the DLL in system directories.
• Tested on Windows with the latest version of the project dependencies.

Let me know if additional clarifications or adjustments are needed!