fix: install script location default & parsing

[ThatOneCalculator]

Why this should be merged

Removes trailing slash from install path (prevents cases like install.sh -b /usr/local/bin/ producing ava-labs/avalanche-cli info installed /usr/local/bin//avalanche)

How this works

• Sets BINDIR to itself but with trailing slash removed if it exists
• Then sets BINDIR to default if it's an empty string

How this was tested

• Ran install script

How is this documented

• Code comments

[ThatOneCalculator]

Requesting review from @felipemadero