NeonEnv.from_repo_dir: use storage_controller_db instead of attachments.json (#8382)

When `NeonEnv.from_repo_dir` was introduced, storage controller stored
its
state exclusively `attachments.json`.
Since then, it has moved to using Postgres, which stores its state in
`storage_controller_db`.

But `NeonEnv.from_repo_dir` wasn't adjusted to do this.
This PR rectifies the situation.

Context for this is failures in
`test_pageserver_characterize_throughput_with_n_tenants`
CF:
https://neondb.slack.com/archives/C033RQ5SPDH/p1721035799502239?thread_ts=1720901332.293769&cid=C033RQ5SPDH

Notably, `from_repo_dir` is also used by the backwards- and
forwards-compatibility.
Thus, the changes in this PR affect those tests as well.
However, it turns out that the compatibility snapshot already contains
the `storage_controller_db`.
Thus, it should just work and in fact we can remove hacks like
`fixup_storage_controller`.

Follow-ups created as part of this work:
* https://github.com/neondatabase/neon/issues/8399
* https://github.com/neondatabase/neon/issues/8400

Cargo.lock

@@ -1368,6 +1368,7 @@ dependencies = [
  "tracing",
  "url",
  "utils",
+ "whoami",
  "workspace_hack",
 ]
 
@@ -4603,6 +4604,15 @@ dependencies = [
  "bitflags 1.3.2",
 ]
 
+[[package]]
+name = "redox_syscall"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4722d768eff46b75989dd134e5c353f0d6296e5aaa3132e776cbdb56be7731aa"
+dependencies = [
+ "bitflags 1.3.2",
+]
+
 [[package]]
 name = "regex"
 version = "1.10.2"
@@ -6972,6 +6982,12 @@ version = "0.11.0+wasi-snapshot-preview1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
 
+[[package]]
+name = "wasite"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8dad83b4f25e74f184f64c43b150b91efe7647395b42289f38e50566d82855b"
+
 [[package]]
 name = "wasm-bindgen"
 version = "0.2.92"
@@ -7124,6 +7140,17 @@ dependencies = [
  "once_cell",
 ]
 
+[[package]]
+name = "whoami"
+version = "1.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a44ab49fad634e88f55bf8f9bb3abd2f27d7204172a112c7c9987e01c1c94ea9"
+dependencies = [
+ "redox_syscall 0.4.1",
+ "wasite",
+ "web-sys",
+]
+
 [[package]]
 name = "winapi"
 version = "0.3.9"