fix(mingw): view only a prefix that holds a Linux system

The first fix showed clang every mingw prefix through a view of its
mingw-w64 parts. That broke a prefix that holds mingw-w64 alone, where
Windows libraries installed with --prefix land in include/ and lib/:
with CRAFTER_MINGW_DIR=/opt/mingw, <SDL2/SDL.h> and -lSDL2 were no
longer found. Such a prefix is clang's sysroot whole again. Only a
prefix that also holds a Linux system gets the view
(Toolchain::mingwPrefixIsSystem): the host's /usr inside the Flatpak,
whatever its include/ holds, and a prefix whose include/ has a C
library's features.h, which every glibc and musl header includes and
mingw-w64 has no use for, as CRAFTER_MINGW_DIR=/usr has.

MakeMingwSysroot makes the view in the cache, or in
$XDG_RUNTIME_DIR/crafter.build when the cache's filesystem takes no
symbolic links (vfat, exFAT, CIFS without unix extensions): the XDG
spec promises them there, and nothing else in the cache needs them.
Its path is absolute, because CMake runs the compiler from directories
of its own and a relative cache did not resolve from there. The prefix
is resolved once, absolute and lexically normal, so /opt/mingw/ and
/opt/mingw are one prefix and not two builds.

clang names a header by the path it found it at, so every diagnostic
from a viewed mingw-w64 header named a hashed cache directory.
RunBuildCommand now writes paths into the view as paths into the
prefix, as BuildStdPcm did for its manifest messages already.

Also from the review of PR #44: SysrootCompileFlags builds a mingw
command, which can make a view, so it redirects the cache like the
other tests; the manifest filter's comment names what it still keeps
out; the C path asks MingwSysroot() once.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/SysrootCompileFlags/main.cpp

@@ -19,6 +19,9 @@ using namespace Crafter;
 // sysroot's copy. It must not do so where there is no sysroot, for wasm (which
 // has its own sysroot handling) or for mingw (libstdc++, no libc++ at all).
 // The command is checked as text: no compiler runs.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
 namespace {
     std::int32_t Failures = 0;
 
@@ -44,6 +47,15 @@ namespace {
 }
 
 int main() {
+    // A compile command names the std module's directory in the cache, and
+    // the mingw one can make the view of a system mingw prefix there
+    // (MingwSysroot), as with CRAFTER_MINGW_DIR=/usr or inside the Flatpak
+    // on a host with mingw-w64. Set before anything resolves the toolchain.
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-sysroot-compile-flags-cache";
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    setenv("XDG_CACHE_HOME", scratch.c_str(), 1);
+
     const std::string sysroot = "/opt/crafter-build-test-sysroot";
     const std::string forced = std::format("-nostdinc++ -isystem {}/usr/include/c++/v1", sysroot);
 
@@ -57,6 +69,7 @@ int main() {
     Check(!CommandFor("wasm32-wasip1", sysroot).contains(forced), "wasm: left to its own sysroot handling");
     Check(!CommandFor("x86_64-w64-mingw32", sysroot).contains(forced), "mingw: libstdc++ target, libc++ headers are not forced");
 
+    fs::remove_all(scratch, ec);
     if (Failures > 0) {
         std::println(std::cerr, "{} assertions failed", Failures);
         return 1;