Don't put CompUnitRepo folders outside the build dir during build

CompUnitRepo generates a folder structure in the PERL6_HOME folder. That
folder does not exist yet during the build. So the normal logic to
auto determine the PERL6_HOME folder thinks it's
`build_dir/../share/perl6`. Thus CompUnitRepo puts some folders there.
This is not wanted. As a temporary solution to prevent pollution of the
users directories, we just explicitly set PERL6_HOME to
`build_dir/gen/dummy_p6_home` then CompUnitRepo can litter it's folders
there and all is well.

tools/templates/moar/perl6-m-build.c.in

@@ -3,6 +3,7 @@
 #include <string.h>
 #include <errno.h>
 #include <unistd.h>
+extern char **environ;
 
 int main(int argc, char *argv[])
 {
@@ -35,7 +36,11 @@ int main(int argc, char *argv[])
 
     exec_argv[exec_argc - 1] = NULL;
 
-    execv(moar, exec_argv);
+    // Hack: Explicitly set a dummy PERL6_HOME, so the autogenerated CompUnitRepo
+    // folder structure does not end up in the build directories parent folder.
+    setenv("PERL6_HOME", "@c_escape(@nfp(@base_dir@/gen/dummy_p6_home)@)@", 1);
+
+    execve(moar, exec_argv, environ);
 
     // execv doesn't return on successful exec.
     fprintf(stderr, "ERROR: Failed to execute moar. Error code: %i\n", errno);

tools/templates/moar/perl6-m-build.c.windows

@@ -120,6 +120,10 @@ int wmain(int argc, wchar_t *argv[]) {
     }
     cmd_line[cmd_line_size - 1] = 0;
 
+    // Hack: Explicitly set a dummy PERL6_HOME, so the autogenerated CompUnitRepo
+    // folder structure does not end up in the build directories parent folder.
+    _wputenv_s(L"PERL6_HOME", L"@c_escape(@nfp(@base_dir@/gen/dummy_p6_home)@)@");
+
     // Execute the command and wait for it to finish.
     ZeroMemory(&si, sizeof(si));
     si.cb = sizeof(si);