[RustPlanner] use InstallStage (#151)

## Summary

We can use the InstallStage as intended: leverage Docker layers to
ensure incremental
builds are faster, when no crates (rust dependencies) change.

## How was it tested?

in `testdata/rust/rust-stable`:
```
> devbox build

> docker run devbox
# printed the version number used.
```

Author: savil

planner/languages/rust/rust_planner.go

@@ -63,9 +63,13 @@ func (p *Planner) getPlan(srcDir string) (*plansdk.Plan, error) {
 		// 2. https://stackoverflow.com/a/56166959
 		DevPackages:     []string{rustPkgDev, "gcc"},
 		RuntimePackages: []string{"glibc"},
+		InstallStage: &plansdk.Stage{
+			InputFiles: []string{"."},
+			Command:    "cargo fetch",
+		},
 		BuildStage: &plansdk.Stage{
 			InputFiles: []string{"."},
-			Command:    "cargo build --release",
+			Command:    "cargo build --release --offline",
 		},
 		StartStage: &plansdk.Stage{
 			InputFiles: []string{fmt.Sprintf("target/release/%s", manifest.PackageField.Name)},

testdata/rust/rust-1.62.0/plan.json

@@ -11,10 +11,13 @@
     "glibc"
   ],
   "install_stage": {
-    "command": ""
+    "command": "cargo fetch",
+    "input_files": [
+      "."
+    ]
   },
   "build_stage": {
-    "command": "cargo build --release",
+    "command": "cargo build --release --offline",
     "input_files": [
       "."
     ]

testdata/rust/rust-stable-hello-world/plan.json

@@ -10,10 +10,13 @@
     "glibc"
   ],
   "install_stage": {
-    "command": ""
+    "command": "cargo fetch",
+    "input_files": [
+      "."
+    ]
   },
   "build_stage": {
-    "command": "cargo build --release",
+    "command": "cargo build --release --offline",
     "input_files": [
       "."
     ]

testdata/rust/rust-stable/plan.json

@@ -11,10 +11,13 @@
     "glibc"
   ],
   "install_stage": {
-    "command": ""
+    "command": "cargo fetch",
+    "input_files": [
+      "."
+    ]
   },
   "build_stage": {
-    "command": "cargo build --release",
+    "command": "cargo build --release --offline",
     "input_files": [
       "."
     ]