feat(nix-instantiate): add --raw flag

The experimental `nix eval` command already supports a `--raw` flag.
This commit implements the same flag for the stable nix-instantiate command.

Until now instructions and scripts that didn't want to rely on experimental
features had to use workarounds such as:

    nix-instantiate --eval <something> | tr -d \"

(which also undesirably also removes double quotation marks within the string), or

    nix-instantiate --eval <something> | jq -j

(which undesirably depends on another package).

Co-authored-by: Silvan Mosberger <silvan.mosberger@tweag.io>

doc/manual/rl-next/nix-instantiate-raw.md

@@ -0,0 +1,8 @@
+---
+synopsis: "`nix-instantiate --eval` now supports `--raw`"
+prs: [12119]
+---
+
+The `nix-instantiate --eval` command now supports a `--raw` flag, when used
+the evaluation result must be a string, which is printed verbatim without
+quotation marks or escaping.

doc/manual/source/command-ref/nix-copy-closure.md

@@ -84,7 +84,7 @@ When using public key authentication, you can avoid typing the passphrase with `
 > Copy GNU Hello from a remote machine using a known store path, and run it:
 >
 > ```shell-session
-> $ storePath="$(nix-instantiate --eval '<nixpkgs>' -I nixpkgs=channel:nixpkgs-unstable -A hello.outPath | tr -d '"')"
+> $ storePath="$(nix-instantiate --eval --raw '<nixpkgs>' -I nixpkgs=channel:nixpkgs-unstable -A hello.outPath)"
 > $ nix-copy-closure --from alice@itchy.example.org "$storePath"
 > $ "$storePath"/bin/hello
 > Hello, world!

doc/manual/source/command-ref/nix-instantiate.md

@@ -5,7 +5,7 @@
 # Synopsis
 
 `nix-instantiate`
-  [`--parse` | `--eval` [`--strict`] [`--json`] [`--xml`] ]
+  [`--parse` | `--eval` [`--strict`] [`--raw` | `--json` | `--xml`] ]
   [`--read-write-mode`]
   [`--arg` *name* *value*]
   [{`--attr`| `-A`} *attrPath*]
@@ -102,6 +102,11 @@ standard input.
   > This option can cause non-termination, because lazy data
   > structures can be infinitely large.
 
+- `--raw`
+
+  When used with `--eval`, the evaluation result must be a string,
+  which is printed verbatim, without quoting, escaping or trailing newline.
+
 - `--json`
 
   When used with `--eval`, print the resulting value as an JSON