Read glibc's version in a locale-independent way

We want to enable posix_spawn only for glibc >= 2.24, so we check
gnu_get_libc_version() at runtime. This returns a string with the
version number.

Because it's a version number it's spelt with a "." and never a ",",
but we interpret it as a float. This is iffy to begin with, but simple
enough. Only when the locale uses a ",", things break - it'll read it
as "2" and fail the check, which absolutely *tanks* performance on WSL1.

I'm unsure if this gives the proper runtime glibc version - it might,
whereas __GLIBC_MINOR__ and such definitely would not.

So fix the immediate problem by at least using a c locale - this is
already masked by 8dc3982408, but better
safe than sorry.

src/env_dispatch.cpp

@@ -280,7 +280,7 @@ static bool allow_use_posix_spawn() {
     // uClibc defines __GLIBC__.
 #if defined(__GLIBC__) && !defined(__UCLIBC__)
     const char *version = gnu_get_libc_version();
-    result = version && strtod(version, nullptr) >= 2.24;
+    result = version && strtod_l(version, nullptr, fish_c_locale()) >= 2.24;
 #endif
     return result;
 }