refactor(tvix): Use SANDBOX_SHELL from environment in CMake

This makes the configuration work out-of-the-box using CMake in a Nix
shell, rather than having to pass the additional variable to CMake on
the command line.

Change-Id: I04d3cc4f2e5ecf47bf2ee459d5e48588b84ae4dd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1643
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/CMakeLists.txt

@@ -40,7 +40,9 @@ if (CLANG_TIDY_PATH)
   endif()
 endif()
 
-if (NOT SANDBOX_SHELL)
+if (DEFINED ENV{SANDBOX_SHELL})
+  set(SANDBOX_SHELL ENV{SANDBOX_SHELL})
+else()
   find_program(BUSYBOX busybox)
   if (BUSYBOX)
     set(SANDBOX_SHELL "${BUSYBOX}")

third_party/nix/default.nix

@@ -95,8 +95,7 @@ in lib.fix (self: pkgs.llvmPackages.libcxxStdenv.mkDerivation {
       -DCMAKE_BUILD_TYPE=RelWithDebInfo \
       -DCMAKE_FIND_USE_SYSTEM_PACKAGE_REGISTRY=OFF \
       -DCMAKE_FIND_USE_PACKAGE_REGISTRY=OFF \
-      -DCMAKE_EXPORT_NO_PACKAGE_REGISTRY=ON \
-      -DSANDBOX_SHELL=${pkgs.busybox}/bin/busybox
+      -DCMAKE_EXPORT_NO_PACKAGE_REGISTRY=ON
   '';
 
   installCheckPhase = ''