Fix malformed installer URLs for Windows 386 (32-bit)

[Copilot]

On Windows with archType=386, getReleaseForWindows produced malformed URLs (python-3.14.0-.exe, python-3.14.0..msi) that 404'd, making 32-bit installs impossible.

Two bugs in lib/util.lua:

• MSI format string had a hardcoded . before the arch placeholder (%s/python-%s.%s.msi), producing a double-dot when arch is empty
• EXE call site unconditionally prepended - to archType ('-' .. archType), producing a trailing dash when arch is empty

Fix

• Remove the embedded dot from the MSI format string: %s/python-%s%s.msi
• Compute full arch suffixes (separator + value, or empty) before formatting:

-- Before
if archType == "386" then archType = "" end
local url = DOWNLOAD_SOURCE.EXE:format(version, version, '-' .. archType)  -- → "python-3.14.0-.exe"
local url = DOWNLOAD_SOURCE.MSI:format(version, version, archType)          -- → "python-3.14.0..msi"

-- After
local exeArchSuffix, msiArchSuffix = "", ""
if archType ~= "386" then
    exeArchSuffix = "-" .. archType   -- e.g. "-amd64"
    msiArchSuffix = "." .. archType   -- e.g. ".amd64"
end
local url = DOWNLOAD_SOURCE.EXE:format(version, version, exeArchSuffix)  -- → "python-3.14.0.exe"
local url = DOWNLOAD_SOURCE.MSI:format(version, version, msiArchSuffix)  -- → "python-3.14.0.msi"

arch	URL (before)	URL (after)
386 EXE	python-3.14.0-.exe ❌	python-3.14.0.exe ✅
386 MSI	python-3.14.0..msi ❌	python-3.14.0.msi ✅
amd64 EXE	python-3.14.0-amd64.exe ✅	python-3.14.0-amd64.exe ✅
amd64 MSI	python-3.14.0.amd64.msi ✅	python-3.14.0.amd64.msi ✅