[FEATURE]: support custom version and binary name via env vars

[mabd-dev]

What problem does this solve?

• Allow installing a specific version instead of always pulling latest
• Allow specifying the installed binary name — useful for keeping a stable
  reposcan alongside a reposcan-dev build to compare behaviour

Proposed solution

Update install.sh to read two optional environment variables:

• VERSION — the release tag to install (default: latest)
• ALIAS — the name to give the installed binary (default: reposcan)

Usage

Default (existing behaviour, unchanged):

curl -fsSL https://raw.githubusercontent.com/mabd-dev/reposcan/main/install.sh | sh

Pin a version:

curl -fsSL https://raw.githubusercontent.com/mabd-dev/reposcan/main/install.sh | VERSION=v1.2.0 sh

Custom binary name:

curl -fsSL https://raw.githubusercontent.com/mabd-dev/reposcan/main/install.sh | ALIAS=reposcan-dev sh

Combined:

curl -fsSL https://raw.githubusercontent.com/mabd-dev/reposcan/main/install.sh | VERSION=v1.2.0 ALIAS=reposcan-dev sh

Implementation checklist

• Skip latest version fetch if VERSION is already set
• Use ALIAS as the installed binary name, defaulting to reposcan
• Update README install instructions with the new optional params