Sandbox the OUT_DIR environment variable

Currently, OUT_DIR is inherited from the parent process, leading to
scripts being able to find the output directory when the enviornment
variable is set to an absolute path. When sandboxing a command,
also rewrite the OUT_DIR environment variable to the sandboxed one,
so that scripts can't find the real out dir.

Bug: 307824623
Test: Presubmits
Change-Id: I325071121a60bddc4105df680fbdfe3d11dc94e2
diff --git a/cmd/sbox/sbox_proto/sbox.proto b/cmd/sbox/sbox_proto/sbox.proto
index 2f0dcf0..1158554 100644
--- a/cmd/sbox/sbox_proto/sbox.proto
+++ b/cmd/sbox/sbox_proto/sbox.proto
@@ -51,6 +51,30 @@
   // A list of files that will be copied before the sandboxed command, and whose contents should be
   // copied as if they were listed in copy_before.
   repeated RspFile rsp_files = 6;
+
+  // The environment variables that will be set or unset while running the command.
+  // Also see dont_inherit_env.
+  repeated EnvironmentVariable env = 7;
+
+  // By default, all environment variables are inherited from the calling process, but may be
+  // replaced or unset by env. If dont_inherit_env is set, no environment variables will be
+  // inherited, and instead only the variables in env will be defined.
+  optional bool dont_inherit_env = 8;
+}
+
+message EnvironmentVariable {
+  // The name of the environment variable
+  required string name = 1;
+  oneof state {
+    // The value to set the environment variable to.
+    string value = 2;
+    // This environment variable should be unset in the command.
+    bool unset = 3;
+    // This environment variable should be inherited from the parent process.
+    // Can be combined with dont_inherit_env to only inherit certain environment
+    // variables.
+    bool inherit = 4;
+  }
 }
 
 // Copy describes a from-to pair of files to copy.  The paths may be relative, the root that they