fix(xanthous/server): Fix decoding secret key

The actual function we want for the format we're using is
decode_secret_key, not decode_openssh, apparently - covered this with a
toneest to make sure.

Change-Id: I659226169f213b8464b96aec6b94bf13fd80aac8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3863
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

users/grfn/xanthous/server/Cargo.lock

@@ -1319,6 +1319,15 @@ version = "0.6.25"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f497285884f3fcff424ffc933e56d7cbca511def0c9831a7f9b5f6153e3cc89b"
 
+[[package]]
+name = "remove_dir_all"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3acd125665422973a33ac9d3dd2df85edad0f4ae9b00dafb1a05e43a9f5ef8e7"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "rustc-demangle"
 version = "0.1.21"
@@ -1456,6 +1465,20 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "tempfile"
+version = "3.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dac1c663cfc93810f88aed9b8941d48cabf856a1b111c29a40439018d870eb22"
+dependencies = [
+ "cfg-if 1.0.0",
+ "libc",
+ "rand",
+ "redox_syscall",
+ "remove_dir_all",
+ "winapi",
+]
+
 [[package]]
 name = "termcolor"
 version = "1.1.2"
@@ -1889,6 +1912,7 @@ dependencies = [
  "metrics-exporter-prometheus",
  "nix",
  "pbkdf2",
+ "tempfile",
  "thrussh",
  "thrussh-keys",
  "tokio",