fix: add context to dfx cache installation message

[ericswanson-dfinity]

Description

When dfx runs for the first time, it installs binaries, including itself, to the dfx cache. It also displays a message that it has done so. Without context, this message can be confusing.

Changed the final message to include context about what (dfx) it's installing and where (the cache)

How Has This Been Tested?

before

$ dfx extension install nns --version 0.3.1
  Version v0.21.0-beta.0+rev12.ecf712721 installed successfully.
  Extension 'nns' installed successfully

after

$ dfx extension install nns --version 0.3.1
  Installed dfx 0.21.0-beta.0+rev12.ecf712721 to cache.
  Extension 'nns' installed successfully

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [ ] I have edited the CHANGELOG accordingly.
• [ ] I have made corresponding changes to the documentation.