Add support for CARGO_MOMMYS_ACTUAL to control child program

It may potentially be desirable to use mommy with not-cargo programs.
While today we can use `CARGO` to control this, this does set the
`CARGO` env var, which may make some tools unhappy (for example, if
another cargo gets called beneath this other program). To avoid this,
this adds a neutral environment variable, `CARGO_MOMMYS_ACTUAL`.

README.md

@@ -53,6 +53,17 @@ mommy knows her little girl can do better~ ❤️
 If you try to make it into an alias, you should prefer pointing it to `cargo-mommy` directly,
 we wouldn't want to break the rustup toolchain picker, now would we?~
 
+If you want to use `cargo-mommy` for not-cargo programs, just set the `CARGO_{ROLE}S_ACTUAL`
+environment variable to it, for example
+
+```
+$ CARGO_MOMMYS_ACTUAL=date ./target/debug/cargo-mommy
+Sun Nov 19 05:33:34 PM CET 2023
+what a good girl you are~
+```
+
+mommy will also respect `CARGO` to execute the right cargo for you~
+
 # Configuration
 
 Mommy will read the following environment variables to make her messages better for you~ ❤️

src/main.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::let_and_return)]
 
 use fastrand::Rng;
+use std::env;
 use std::io::IsTerminal;
 
 enum ResponseType {
@@ -29,8 +30,8 @@ fn main() {
 
 fn real_main() -> Result<i32, Box<dyn std::error::Error>> {
     let rng = Rng::new();
-    let cargo = std::env::var("CARGO").unwrap_or_else(|_| "cargo".to_owned());
-    let mut arg_iter = std::env::args().peekable();
+
+    let mut arg_iter = env::args().peekable();
 
     // Understand who mommy really is~
     //
@@ -43,7 +44,7 @@ fn real_main() -> Result<i32, Box<dyn std::error::Error>> {
 
     // Interpret the argument as a path so we can manipulate it~
     let first_arg = arg_iter.next();
-    let bin_path = std::env::current_exe()
+    let bin_path = env::current_exe()
         .unwrap_or_else(|_| std::path::PathBuf::from(first_arg.unwrap_or_default()));
     // Get the extensionless-file name, and parse it case-insensitively~
     let bin_name = bin_path
@@ -58,10 +59,14 @@ fn real_main() -> Result<i32, Box<dyn std::error::Error>> {
         "mommy".to_owned()
     };
 
+    let cargo = env::var(format!("CARGO_{}S_ACTUAL", true_role.to_uppercase()))
+        .or_else(|_| env::var("CARGO"))
+        .unwrap_or_else(|_| "cargo".to_owned());
+
     // Check if someone has told mommy to keep calling herself~
     // Mommy loves you, darlings, but she can't keep running forever~
     let mut new_limit = 1;
-    if let Ok(limit) = std::env::var(RECURSION_LIMIT_VAR) {
+    if let Ok(limit) = env::var(RECURSION_LIMIT_VAR) {
         if let Ok(n) = limit.parse::<u8>() {
             if n > RECURSION_LIMIT {
                 let mut response = select_response(&true_role, &rng, ResponseType::Overflow);
@@ -294,7 +299,7 @@ impl Var<'_> {
     /// produces an in-character error message on failure.
     fn load(&self, true_role: &str, rng: &Rng) -> Result<String, String> {
         // try to load custom settings from env vars~
-        let var = std::env::var(self.env(true_role));
+        let var = env::var(self.env(true_role));
         let split;
 
         // parse the custom settings or use the builtins~