Make sure we have an execvpe on Windows too

Necessary to fix a build (that was already broken in other ways) after
PR #11021.

src/libutil/exec.hh

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "os-string.hh"
+
 namespace nix {
 
 /**
@@ -8,6 +10,9 @@ namespace nix {
  *
  * We use our own implementation unconditionally for consistency.
  */
-int execvpe(const char * file0, char * const argv[], char * const envp[]);
+int execvpe(
+    const OsString::value_type * file0,
+    const OsString::value_type * const argv[],
+    const OsString::value_type * const envp[]);
 
 }

src/libutil/meson.build

@@ -129,6 +129,7 @@ sources = files(
   'english.cc',
   'environment-variables.cc',
   'error.cc',
+  'exec.hh',
   'executable-path.cc',
   'exit.cc',
   'experimental-features.cc',

src/libutil/unix/meson.build

@@ -13,7 +13,6 @@ sources += files(
 include_dirs += include_directories('.')
 
 headers += files(
-  'exec.hh',
   'monitor-fd.hh',
   'signals-impl.hh',
 )

src/libutil/unix/processes.cc

@@ -420,10 +420,12 @@ bool statusOk(int status)
     return WIFEXITED(status) && WEXITSTATUS(status) == 0;
 }
 
-int execvpe(const char * file0, char * const argv[], char * const envp[])
+int execvpe(const char * file0, const char * const argv[], const char * const envp[])
 {
-    auto file = ExecutablePath::load().findPath(file0).string();
-    return execve(file.c_str(), argv, envp);
+    auto file = ExecutablePath::load().findPath(file0);
+    // `const_cast` is safe. See the note in
+    // https://pubs.opengroup.org/onlinepubs/9799919799/functions/exec.html
+    return execve(file.c_str(), const_cast<char *const *>(argv), const_cast<char *const *>(envp));
 }
 
 }

src/libutil/windows/processes.cc

@@ -1,6 +1,7 @@
 #include "current-process.hh"
 #include "environment-variables.hh"
 #include "error.hh"
+#include "executable-path.hh"
 #include "file-descriptor.hh"
 #include "file-path.hh"
 #include "signals.hh"
@@ -377,4 +378,11 @@ bool statusOk(int status)
 {
     return status == 0;
 }
+
+int execvpe(const wchar_t * file0, const wchar_t * const argv[], const wchar_t * const envp[])
+{
+    auto file = ExecutablePath::load().findPath(file0);
+    return _wexecve(file.c_str(), argv, envp);
+}
+
 }