perf(exec): improve performance (#433)

...by only parsing installed packages for their environment variables if
we are in a deactivated hermit environment. When activated, the parent
shell will have already passed the new process all environment
variables, including packages that were installed after initial
activation. Parsing the HCL configs of all installed packages introduces
some overhead. This can be non-negligible when hermit-managed packages
are used a large number of times - for instance with javac or rustc.

Here is a very exaggerated example on a project with 42 installed
packages:

Before change:
```bash
$ time make --version > /dev/null

real    0m0.143s
user    0m0.136s
sys     0m0.033s
```

After change:
```bash
$ time make --version > /dev/null

real    0m0.013s
user    0m0.013s
sys     0m0.000s
```

No hermit:
```bash
$ time /usr/bin/make --version > /dev/null

real    0m0.002s
user    0m0.001s
sys     0m0.001s
```

env.go

@@ -49,7 +49,7 @@ var (
 	//go:embed "files/install.sh.tmpl"
 	InstallerTemplateSource string
 
-	// UserStateDir should be passed to Open()/Init() in most cases.
+	// UserStateDir should be passed to OpenEnv()/Init() in most cases.
 	UserStateDir = func() string {
 		// Check if state dir is explicitly set
 		explicit := os.Getenv("HERMIT_STATE_DIR")
@@ -114,6 +114,7 @@ type Env struct {
 	configFile      string
 	httpClient      *http.Client
 	scriptSums      []string
+	activated       bool
 
 	// Lazily initialized fields
 	lazyResolver  *manifest.Resolver
@@ -349,6 +350,7 @@ func OpenEnv(
 		ephemeralEnvars: envars.Infer(ephemeral.System()),
 		httpClient:      httpClient,
 		scriptSums:      scriptSums,
+		activated:       os.Getenv("DEACTIVATED_HERMIT") == "",
 	}, nil
 }
 
@@ -783,10 +785,21 @@ func (e *Env) Exec(l *ui.UI, pkg *manifest.Package, binary string, args []string
 		return errors.WithStack(err)
 	}
 
-	installed, err := e.ListInstalled(l)
-	if err != nil {
-		return errors.WithStack(err)
+	var installed []*manifest.Package
+
+	// If we are activated, the parent shell already has
+	// already passed us all necessary environment variables.
+	// This may not be the case if the user manually ran
+	// e.g. ./bin/go
+	// We still need to call e.allEnvarOpsForPackages to ensure
+	// the selected package's env vars take precedence.
+	if !e.activated {
+		installed, err = e.ListInstalled(l)
+		if err != nil {
+			return errors.WithStack(err)
+		}
 	}
+
 	ops := e.allEnvarOpsForPackages(runtimeDeps, pkg, installed...)
 	packageHermitBin, err := e.getPackageRuntimeEnvops(pkg)
 	if err != nil {
@@ -797,9 +810,6 @@ func (e *Env) Exec(l *ui.UI, pkg *manifest.Package, binary string, args []string
 	}
 	env := e.envarsFromOps(true, ops)
 
-	if err != nil {
-		return errors.WithStack(err)
-	}
 	for _, bin := range binaries {
 		if filepath.Base(bin) != filepath.Base(binary) {
 			continue