[antlir2][rpms] prevent /dev/null from existing

Summary:
We need to make sure shitty rpm scripts aren't trying to write to `/dev/null`
(and possibly friends).

Test Plan:
```
❯ buck2 build --show-output fbcode//tupperware/image/base/impl:base.c9.os[debug][package_manager][subvol]
BUILD SUCCEEDED
fbcode//tupperware/image/base/impl:base.c9.os[debug][package_manager][subvol] buck-out/v2/gen/fbcode/f88b3f368c9334db/tupperware/image/base/impl/__base.c9.os__/subvol-package_manager_compile

vmagro@devvm11640.ftw0 in fbsource
❯ ls buck-out/v2/gen/fbcode/f88b3f368c9334db/tupperware/image/base/impl/__base.c9.os__/subvol-package_manager_compile/dev
```

Reviewed By: epilatow

Differential Revision: D49062146

fbshipit-source-id: f4327d922a3ef1eb2fbe7f7d467b5e11299796cb

antlir/antlir2/features/BUCK

@@ -95,6 +95,7 @@ feature_impl(
 feature_impl(
     name = "rpm",
     deps = [
+        "nix",
         "serde_json",
         "//antlir/buck/buck_label:buck_label",
     ],

antlir/antlir2/features/rpm.rs

@@ -23,6 +23,8 @@ use anyhow::Context;
 use anyhow::Error;
 use anyhow::Result;
 use buck_label::Label;
+use nix::mount::MntFlags;
+use nix::mount::MsFlags;
 use serde::de::Error as _;
 use serde::Deserialize;
 use serde::Serialize;
@@ -330,6 +332,16 @@ fn run_dnf_driver(
     })
     .context("while serializing dnf-driver input")?;
 
+    std::fs::create_dir_all(ctx.dst_path("/dev")).context("while ensuring /dev exists")?;
+    nix::mount::mount(
+        Some("devtmpfs"),
+        &ctx.dst_path("/dev"),
+        Some("devtmpfs"),
+        MsFlags::empty(),
+        None::<&str>,
+    )
+    .context("while mounting /dev in installroot")?;
+
     let mut child = Command::new("/__antlir2__/dnf/driver")
         .arg(&input)
         .stdout(Stdio::piped())
@@ -344,6 +356,10 @@ fn run_dnf_driver(
         events.push(event);
     }
     let result = child.wait().context("while waiting for dnf-driver")?;
+
+    nix::mount::umount2(&ctx.dst_path("/dev"), MntFlags::empty())
+        .context("while unmounting /dev from installroot")?;
+
     if !result.success() {
         Err(Error::msg("dnf-driver failed"))
     } else {