Ensure rpmbuild can find spec files in subdirs

Resolves https://github.com/naveenrajm7/rpmbuild/issues/7

Previously,

```js
await exec.exec(
  `rpmbuild -ba /github/home/rpmbuild/SPECS/${specFile}`
);
```

would fail when given this input:

```yml
- name: build RPM package
  id: my-rpm
  uses: naveenrajm7/rpmbuild
  with:
    spec_file: "rpm/my-specfile.spec"
```

The failure arises because previously, the code said:

```js
const specFile = core.getInput('spec_file');
await exec.exec(`cp /github/workspace/${specFile} /github/home/rpmbuild/SPECS/`);
```

Which meant the commands which ended up being run in the rpmbuild-container
were equivalent to:

```sh
$ cp /github/workspace/rpm/my-specfile.spec /github/home/rpmbuild/SPECS/
$ rpmbuild -ba /home/github/home/rpmbuild/SPECS/rpm/my-specfile.spec
```

`rpmbuild -ba` fails in this situation because the `cp` does not copy the specfile
into `/github/home/rpmbuild/SPECS/rpm/my-specfile.spec`, but instead
into `/github/home/rpmbuild/SPECS/my-specfile.spec`.

The solution is to use the basename of the specfile for `rpmbuild -ba`.

As a matter of personal preference, I've changed the shell commands to
always operate on source-file and destination-file, rather than on
source-file and destination-paths. I've just generally found my own scripts
easier to maintain when I'm a little more explicit like this.

src/main.ts

@@ -4,6 +4,7 @@ const exec = require('@actions/exec');
 const io = require('@actions/io');
 const cp = require('child_process');
 const fs = require('fs');
+const path = require('path');
 
 async function run() {
   try {
@@ -18,10 +19,15 @@ async function run() {
 
     // get inputs from workflow
     // specFile name
-    const specFile = core.getInput('spec_file');
+    const configPath = core.getInput('spec_file'); // user input, eg: `foo.spec' or `rpm/foo.spec'
+    const basename = path.basename(configPath); // always just `foo.spec`
+    const specFile = {
+      srcFullPath: `/github/workspace/${configPath}`,
+      destFullPath: `/github/home/rpmbuild/SPECS/${basename}`,
+    };
 
     // Read spec file and get values 
-    var data = fs.readFileSync(specFile, 'utf8');
+    var data = fs.readFileSync(specFile.srcFullPath, 'utf8');
     let name = '';       
     let version = '';
 
@@ -41,7 +47,7 @@ async function run() {
     await exec.exec('rpmdev-setuptree');
 
     // Copy spec file from path specFile to /github/home/rpmbuild/SPECS/
-    await exec.exec(`cp /github/workspace/${specFile} /github/home/rpmbuild/SPECS/`);
+    await exec.exec(`cp ${specFile.srcFullPath} ${specFile.destFullPath}`);
 
     // Make the code in /github/workspace/ into a tar.gz, located in /github/home/rpmbuild/SOURCES/
     const oldGitDir = process.env.GIT_DIR;
@@ -52,7 +58,7 @@ async function run() {
     // Execute rpmbuild , -ba generates both RPMS and SPRMS
     try {
       await exec.exec(
-        `rpmbuild -ba /github/home/rpmbuild/SPECS/${specFile}`
+        `rpmbuild -ba ${specFile.destFullPath}`
       );
     } catch (err) {
       core.setFailed(`action failed with error: ${err}`);