Add QSV_DOTENV_PATH env var

This allows qsv to load env vars from a specified dotenv file

docs/ENVIRONMENT_VARIABLES.md

@@ -2,6 +2,7 @@
 
 | Variable | Description |
 | --- | --- |
+| `QSV_DOTENV_PATH` | The filename of the dotenv file to load, OVERRIDING existing environment variables. This takes precedence over any other dotenv file in the filesystem. |
 | `QSV_DEFAULT_DELIMITER` | single ascii character to use as delimiter.  Overrides `--delimiter` option. Defaults to "," (comma) for CSV files & "\t" (tab) for TSV files when not set. Note that this will also set the delimiter for qsv's output to stdout.<br>However, using the `--output` option, regardless of this environment variable, will automatically change the delimiter used in the generated file based on the file extension - i.e. comma for `.csv`, tab for `.tsv` & `.tab` files. |
 | `QSV_SNIFF_DELIMITER` | if set, the delimiter is automatically detected. Overrides `QSV_DEFAULT_DELIMITER` & `--delimiter` option. Note that this does not work with stdin. |
 | `QSV_NO_HEADERS` | if set, the first row will **NOT** be interpreted as headers. Supersedes `QSV_TOGGLE_HEADERS`. |
@@ -54,7 +55,8 @@ qsv supports the use of `.env` files to set environment variables. The `.env` fi
 
 It processes `.env` files as follows:
 
-* Upon invocation, qsv will look for a file named `.env` in the current working directory. If one is found, it will be processed.
+* Upon invocation, qsv will check if the `QSV_DOTENV_PATH` environment variable is set. If it is, it will look for the file specified by the variable. If the file is found, it will be processed.
+* If the `QSV_DOTENV_PATH` environment variable is not set, qsv will look for a file named `.env` in the current working directory. If one is found, it will be processed.
 * If no `.env` file is not found in the current working directory, qsv will next look for an `.env` file with the same filestem as the binary in the directory where the binary is (e.g. if `qsv`/`qsvlite`/`qsvdp` is in `/usr/local/bin`, it will look for `/usr/local/bin/qsv.env`, `/usr/local/bin/qsvlite.env` or `/usr/local/bin/qsvdp.env` respectively).
 * If no `.env` files are found, qsv will proceed with its default settings and the current environment variables, which may include "QSV_" variables.
 

src/util.rs

@@ -1216,12 +1216,24 @@ pub fn transform(bs: &[u8], casei: bool) -> ByteString {
 }
 
 pub fn load_dotenv() -> CliResult<()> {
-    // Use the default .env file in the current directory.
+    // First, check if there is a QSV_DOTENV_PATH environment variable set
+    // if there is, use that as the .env file.
+    // Second, use the default .env file in the current directory.
     // If there is no .env file in the current directory, check if there is
     // an .env file with the same filestem as the binary, in the same directory as the binary.
     // If there is, use that. Failing that, qsv proceeds with its default settings and
     // whatever manually set environment variables are present.
 
+    if let Ok(dotenv_path) = std::env::var("QSV_DOTENV_PATH") {
+        if let Err(e) = dotenvy::from_filename_override(dotenv_path.clone()) {
+            return fail_clierror!(
+                "Cannot process .env file set in QSV_DOTENV_PATH - {dotenv_path}: {e}"
+            );
+        }
+        log::info!("Using .env file: {dotenv_path}");
+        return Ok(());
+    }
+
     // check if there is an .env file in the current directory
     if dotenvy::dotenv_override().is_ok() {
         log::info!(