Fix `bin/fetch-proxy` on Linux (#4117)

`bin/fetch-proxy` was failing on Linux:

```bash
$ bin/fetch-proxy
linkerd2-proxy-v2.87.0/
linkerd2-proxy-v2.87.0/LICENSE
linkerd2-proxy-v2.87.0/bin/
linkerd2-proxy-v2.87.0/bin/linkerd2-proxy
bin/fetch-proxy: 31: [: Linux: unexpected operator
/home/siggy/code/linkerd2/target/proxy/linkerd2-proxy-v2.87.0
```

Also in CI:
https://github.com/linkerd/linkerd2/runs/473746447?check_suite_focus=true#step:5:32

Unfortunately `bin/fetch-proxy` still returned a zero exit status, because
`set -e` does not apply to commands that are part of `if` statements.
From https://ss64.com/bash/set.html:
```
-e  Exit immediately if a simple command exits with a non-zero status, unless
    the command that fails is part of an until or  while loop, part of an
    if statement, part of a && or || list, or if the command's return status
    is being inverted using !.  -o errexit
```
Fortunately when the `if` command failed, it fell through to the `else` clause
for Linux, and copied `linkerd-proxy` successfully.

Root cause was a `==` instead of `=`. `shellcheck` confirms, and also
recommends quoting:

```bash
$ shellcheck bin/fetch-proxy

In bin/fetch-proxy line 31:
if [ $(uname) == "Darwin" ]; then
     ^-- SC2046: Quote this to prevent word splitting.
              ^-- SC2039: In POSIX sh, == in place of = is undefined.
```

Apply `shellcheck` recommendations.

Signed-off-by: Andrew Seigner <siggy@buoyant.io>

bin/fetch-proxy

@@ -28,7 +28,7 @@ curl -sLO "$assetbase/$shafile"
 
 tar -zxvf "$pkgfile" >&2
 expected=$(awk '{print $1}' "$shafile")
-if [ $(uname) == "Darwin" ]; then
+if [ "$(uname)" = "Darwin" ]; then
   computed=$(openssl dgst -sha256 "$pkgfile" | awk '{print $2}')
 else
   computed=$(sha256sum "$pkgfile" | awk '{print $1}')