chore(asdf2devbox): Only update to latest if no previous version is i…

…nstalled

# Issue

If a version of a program was not packaged in NixOS, `asdf2devbox` would
update to latest which might increase the version skew.

E.g. Go 1.22.4 is not packaged in NixOS, updating to it would update to
`go@latest` which might be Go 1.23 - in effect a major version upgrade.

# Fix

asdf2devbox does not update if a version was already referenced in
devbox, to minimize version skew.

scripts/asdf2devbox.py

@@ -43,9 +43,10 @@ def get_installed_version(package):
                 try:
                     subprocess.run(['devbox', 'add', f"{program}@{version}"], check=True)
                 except subprocess.CalledProcessError:
-                    # Fallback to latest in case the exact version is not available
-                    subprocess.run(['devbox', 'add', f"{program}@latest"], check=True)
-                    print(f"Could not find {program}@{version}, using latest instead:")
-                    subprocess.run(['devbox', 'info', program], check=True)
+                    # Fallback to latest in case the exact version is not available and there's no previous version installed
+                    if installed_version is None:
+                        print(f"Could not find {program}@{version}, using latest instead")
+                        subprocess.run(['devbox', 'add', f"{program}@latest"], check=True)
+                        subprocess.run(['devbox', 'info', program], check=True)
             else:
                 print(f"{program}@{version} is already installed")