fix(openssl/windows): capture the VS environment instead of calling vcvars

Running vcvars in a child cmd did not help — the log gained '[bat] handing
build to child cmd' and vcvars' own success banner, and then stopped again with
no RESULT from the OUTER script either. Whatever vcvars does on exit here, it
is not something a caller can survive, in this process or a child of it.

So it is no longer called. It now runs in a cmd whose only job is to dump the
environment it produced, and those variables are imported into the build script:

    cmd /c ""%MCPP_VCVARS%" & set" > vsenv.txt
    for /f "tokens=1* delims==" %%a in (vsenv.txt) do set "%%a=%%b"

This is the standard way build systems capture a Visual Studio environment, and
it removes the dependency on vcvars returning to anybody. `&` is deliberate
rather than `&&`: `set` has to run whatever exit status vcvars leaves behind.

The dump also becomes evidence in its own right — if the toolset is not really
there, the file is missing or short, and that is now a named failure (13) rather
than a silent stop.

Author: Sunrisepeak

pkgs/c/compat.openssl.lua

@@ -476,12 +476,21 @@ local function _install_windows_impl()
     local logw  = tostring(logf):gsub("/", "\\")
     local prefw = tostring(prefix):gsub("/", "\\")
     local innerw = tostring(inner):gsub("/", "\\")
+    local envdump = path.join(prefix, "mcpp_vsenv.txt")
+    local envw = tostring(envdump):gsub("/", "\\")
     io.writefile(inner, table.concat({
         "@echo off",
-        -- vcvars runs HERE, in a child cmd, so whatever it does to its caller
-        -- cannot reach the outer script.
-        'call "%MCPP_VCVARS%" >> "' .. logw .. '" 2>&1',
-        'if errorlevel 1 exit /b 13',
+        -- vcvars is never `call`ed. Three runs showed the caller vanishing the
+        -- moment it finished — even from a child cmd — so instead it runs in a
+        -- cmd whose only job is to dump the resulting environment, and those
+        -- variables are imported here. This is the standard way build systems
+        -- capture a VS environment, and it does not depend on vcvars returning
+        -- to anyone. Note `&` rather than `&&`: `set` must run whatever exit
+        -- status vcvars leaves behind.
+        'echo [bat] capturing VS environment >> "' .. logw .. '" 2>&1',
+        'cmd /c ""%MCPP_VCVARS%" & set" > "' .. envw .. '" 2>>"' .. logw .. '"',
+        'if not exist "' .. envw .. '" ( echo [bat] no env dump produced >> "' .. logw .. '" & exit /b 13 )',
+        'for /f "usebackq tokens=1* delims==" %%a in ("' .. envw .. '") do set "%%a=%%b"',
         'echo [bat] toolset ready >> "' .. logw .. '" 2>&1',
         'cd /d "' .. srcroot .. '"',
         'if errorlevel 1 exit /b 14',